electrical safety in grounded, resistance grounded and ug systems

BEN

DER

Electrical Safety in Grounded, Resistance Grounded and Ungrounded Systems A Detailed Guideline for Installers, System Designers and Technical Personnel

Ground faults in modern power systemsAn introduction into the basics

Introduction

Hazardous ground faults occur frequently in today’s electrical wiring. There are multiple ways to describe a ground fault. Please review some typical expressions below:

• A leakage current from a conductor to a frame or ground measured in milli-amperes or amperes

• An insulation breakdown measured in ohms or kilo ohms • A charging current leaking into the ground (Capacitive leak)

Why do we have different terms to describe similar occurrences? What kind of current can be expected in a power system? What does the resistor do in a resistance grounded power system? What kind of monitor does the Bender company recommended for my system?

These questions are generated due to the variety of power systems employed in the field. There is a huge variety of relays on the market to protect these systems. This booklet has been written to provide a brief introduction to the major power systems and the devices manufactured by the Bender company which are suited best to protect these systems in case of a ground fault. The calculations made in the following examples were based on simple formulas assuming test bench or ideal conditions. Values were chosen randomly to support the basic ideas and fundamentals. The following information might be used as a guideline for an integrator or as a reference for a system designer who is facing the first hurdle of identifying a product for a specific ground fault problem. The text was written in a way that both, technical and non-technical personnel can benefit from the information.

Data sheets for the devices recommended in this booklet can be downloaded at:

www.bender.org -> Go to Products -> click on the pdf files

This booklet covers three basic power systems and their protective devices.

Grounded - Resistance grounded -Ungrounded

The information in this booklet was carefully prepared and is believed to be correct, but Bender makes no warranties respecting it and disclaims any responsibility or liability of any kind for any loss or damage as a consequence of anyone’s use of or reliance upon such information.

The rights to modifications are reserved.

2

Introduction

Table of contents:

Different Technologies for different power systems

Basic Power Systems:

- Grounded

The Ground Fault Grounded AC systems 60 cycles GF Interrupting - Shunt trip breaker GF Interrupting - Contactor Grounded AC systems with VFDs Grounded DC systems Locating ground faults -with portable equipment -with a current clamp -with fixed equipment

- Resistance Grounded Resistance Grounded AC systems GFGC - Ground Fault Ground Check Resistance Grounded AC system Ground Fault NGR Monitor

Locating Ground Faults

- Ungrounded or Floating

Insulation Monitoring Device IMD’s - Passive IMD’s - Active IMD’S

Ungrounded systems - Locating ground faults with portable equipment - Story: Ground Fault location in a 480V delta fed system - Locating ground faults with fixed equipment

EDS 470 Ground Fault Location System / EDS 473 Ground Fault Location System

- Off-line Monitoring - with Bender IR...devices The Off-line IMD’s and their measuring voltages

SELECTION GUIDE for Ground Fault Relays

3

Table of Contents

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121314

15

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20-2122-23

242526

27

28

29

30

Different technologies for different power systems Not every monitor or relay will work on any power system. A ground fault relay (GFR) in combination with a current transformer (CT) will work on grounded or resistance grounded systems, but will need very spe-cial consideration if employed in a fl oating system. An Insulation Monitoring Device (IMD) will work very nicely in a fl oating system, but will do nothing but false tripping in a grounded system.

Case # 1The Insulation monitoring device (IMD) installation below will not work out! The IMD (Online megger) will send a measuring signal into a three phase system. The signal will immediately fi nd the neutral ground bonding jumper and indicate a ground fault.

4

3Ph motorSupply side 480VAC

Ground

L1

L2

L3

Schematic: The ungrounded system with RCM technology

C1 C2 C3

Leakage capacitance

GF = i.e. Dead short only mA currents

RCM470 GF relayNo currentreturn path

Case # 2The ground fault relay (GFR) installation below will not work out! The current transformer (CT) will need to see a serious amount of ground fault current. A fl oating delta normally does not create the fault cur-rent magnitude needed since it does not provide a low ohmic neutral ground return path. The device will never trip, not even if there would be a dead short to ground for a couple days.

General Information

Neutral Ground (NG)Jumper

WRONG!

WRONG!

Grounded Systems

5

Supply side 480/277VAC

Ground

L1

L2

L3

Schematic: The grounded system

V V3G= 277V

V

V

V

V12= 480V

V13= 480V

V23= 480V

V VNG= 0V

3Ph motor

Basic power systems - Grounded

Grounded systems are derived from a power source where the neutral is solidly tied to ground via a ground neutral bonding jumper (NG). Often encountered is the typical three phase 208/120V Y or 480/277V Y configuration. Another possibility is a single phase transformer where the neutral is tied into ground or sometimes, in very rare occasions, we might encounter corner grounded deltas. The general population is very familiar with solidly grounded systems due to the fact that nearly every residency in the U.S. is derived from a 240/120V transformer with center tap. The center tap is bonded solidly to ground.

As always, there are advantages and disadvantages to the grounded power system. One disadvantage is the high amount of possible fault current in a ground fault situation. Fire damage or personnel injury can occur. Nevertheless, a tripped over current breaker or a GFCI will enable the electrician to quickly iden-tify a faulty branch. Action will often be taken after a fault has occurred. Preventative maintenance is not necessarily associated with the grounded system.

Grounded Systems

The ground fault

The magnitude of a ground fault current in a solidly grounded system can be very high. Its magnitude depends on the system voltage and the resistance of the ground fault causing part itself. The ground fault current can easily reach a value which is multiple times higher than the nominal load current. A simplified calculation will explain how the high amounts of current are generated:

Please review the schematic on the bottom. The current IF is defined as:

IF = V3G/(RGF + RGR + RNG)

IF Fault Current V3G Voltage between faulted phase and groundRGF Resistance value at shorted pointRGR Resistance of ground pathRNG Resistance of neutral ground bonding jumper(Please note: The resistances were randomly chosen)

IF = 277V/(0.1Ohm + 0.2 Ohm + 0.1 Ohm) = 692.5A

It can be shown with this example calculation that a theoretical fault current will be devastating if a dead short occurs. Nevertheless, a ground fault relay or over current protective devices should trip immedi-ately and interrupt power from the load. How many Amps would flow if a human would touch the same circuit? Answer: Replace the dead short value of 0.1 Ohm with a more realistic figure for a human body part. Lets assume that a person is resting on the frame of a motor. We assume 1000 Ohms of resis-tance from phase L3 and a human body.

IF = 277V/(1000 Ohm + 0.2 Ohm + 0.1 Ohm) = 0.277A = 277 mA


Ground

L1

L3

Schematic: The grounded system with single ground fault

V V3G= 277V

3Ph motor

GF = i.e. Dead shortResistance = 0.1 Ohm

Fault current path via groundResistance assumed 0.2 Ohms

NG pathResistance assumed 0.1 Ohms

R = 1000 ohms

The current is a multiple of 15 mA, which is considered to be the let-go value for humans. 50 mA is consid-ered to be lethal.

6

Grounded Systems

Grounded AC systems 60 cycles - The ground fault device

Most technicians are very familiar with a current transformer based ground fault current relay. Even non technical personnel encounter them on a daily basis in public rest rooms protecting a wall outlet in a wet area. The operating theory behind the relay is as follows. A current transformer (CT) or “donut” is placed around the power wires leading to the protected load. It is important that hot and neutral wires are fed through the CT. This goes for both, single phase and three phase systems. One might come across a three phase system without a neutral, feeding a pump or an industrial motor. In this case the three phases only will be fed through the CT. Basic rule for three phase systems: If the neutral is carried out to the load -> Feed it through the CT. If there is no neutral -> Then do not worry about it. The current transformer will always read zero current in a healthy system even under a full load condition. In accordance to Kirchhoffs laws, Incoming and Outgoing currents will cancel each other out. Assume a 10A load connected to a 480/277VAC system. 10A will be fed from the source into the load, therefore 10A will have to return from the load back to the source. The CT will measure both simultaneously since it is placed around all conductors. The values were randomly chosen. Here is what the CT would see at a specific moment in time:

In accordance with the schematic below: 10A - 5A -5A= 0A for a healthy system A ground fault (lets assume 1A) will divert some of the current from the arrangement and bypass the CT via the ground wire, a frame or the building ground and return back to the source. The new equation for the CT is now: 10A - 5A - 4A = 1A whereas 10A go into the load, 9A return to the source via the phase L2 and L3 and 1 A returns to the source via the ground wire. The CT will step the current (1A) down and forward it to the Ground Fault Relay (GFR). The GFR will then alarm when its set point has been increased. The GF relay in combination with a zero sequence CT will work in resistance grounded systems as well. It will run into its limitations in circuits where wave form modifying equipment, such as Variable Frequency Drives (VFDs) or rectifier components are installed.

7


Ground

L1

L3

Schematic: The grounded system with single ground fault and GF relay

3Ph motor

IF = 1A

I3 = 4A

I2 = 5A

I1 = 10A

GF relay withsummation transformer

Relays: RCM460 series RCM470 series

Grounded Systems

GF interrupting - Shunt trip breaker

Grounded and resistance grounded systems often require not only to be monitored but to be interrupted as well. The ground fault has to be sensed and power has to be removed ASAP (often in milli seconds). Trip levels for the GFCIs (Ground Fault Circuit Interrupters) vary from application to application. Person-nel protection is considered to be at 6mA in the U.S. Equipment protection can be found anywhere from 10mA up to multiple Amps. Industrial branch or load protection can often be seen at 5Amps. Service entrance protection is most likely set to trip at levels in the multiple hundreds of Amps. Below is a wiring schematic outlining the connections between a typical Bender RCM460 or RCM470, a shunt trip breaker and a three phase load. A1, A2 = External power supply; K,L = Connection to the CT; 11,12 = alarm contacts will close and apply 120VAC to the shunt trip in case of an increased set point, the shunt trip breaker will interrupt power to the load. Please note: The shunt trip breaker will have to be manually reset after a trip.


Ground

L1

L2

L3

Schematic: The RCM470 connected to a shunt trip in a resistance grounded system

3Ph motor

A1 A2

K L 11 12 14

~120 VAC

Shunt tripbreaker

RCM 470GF RELAYin failsafe mode

8

Grounded Systems


Ground

L1

L2

L3

Schematic: The RCM470 connected to a contactor in a resistance grounded system

3Ph motor

A1 A2

K L 11 12 14

~120 VAC

Contactor

RCM 470GF RELAY

GF interrupting - Contactor

The wiring schematic on page 8 employed a shunt trip breaker for interrupting purposes. A contactor in combination with the Bender RCM460 or RCM470 can accomplish the same task. Please review the schematic below:A1, A2 = External power supply; K,L = Connection to the CT; 11,14 = alarm contacts will open and remove power (120VAC) from the contactor holding coil in case of an increased set point. The contactor will drop out and interrupt power to the load. Please note: The contactor can be automatically reset after the fault has been cleared.

9

Grounded Systems

10

Grounded AC systems with VFDs

The 60 cycle GFRs have limitations when the circuitry involves VFDs (Variable Frequency Drives). Tests have shown that the typical GFR cannot keep the adjusted trip point when the system frequency changes to values below 60 cycles. Even worse, a total failure can be expected at frequencies below 12 cycles. A variable frequency drive converts the incoming AC internally into DC, which will then be modulated again into a variable cycle AC leading to the load. Internal VFD - DC grounds can not be detected with conven-tional GFR technology. The common “passive” CT needs alternating currents to detect a ground fault, therefore DC currents will go unnoticed. Some drives might be equipped with their own internal scheme to detect ground faults which will eventually trip in the high Ampere range. Early warning or personnel protection cannot be guaranteed in this case.

Other issues with VFDs:

- The VFD often incorporates built in EMI filters. They provide a leakage path to ground and add to the overall system leakage. - The drive uses a multiple KHZ carrier frequency. The carrier frequency can cross the gap between insulation and ground and add to the inherent leakage. - Harmonic content - Transient voltage spikes

The solution: Grounded systems with VFDs have to be protected by means of “active” current transform-ers with built in filter technology. The Bender RCMA470 in combination with the active CT employs a double coil system which enables the unit to measure accurate AC, DC and mixed AC/DC currents from 0 to 700Hz. Its trip settings range from 6mA up to 3A. It can therefore be used for personnel and branch/motor protection.

AC Leakage

DC linkRectifierEMI filter IGBT

U

V

W

DC Leakage

Variable Frequency Drive

Supply side 480/277VACL1

L3

Schematic: The grounded AC system with a variable frequency drive (VFD) and RCMA technology

3Ph motor

RCMA relay withactive transformer

Possible AC fault 60Hz Capacitive leakage throughEMI filters

Internal DC fault Variable cycle AC fault

L2

Relays: RCMA470 (Motor protection) RCMA473 (Personnel)

Grounded Systems

11

DC systems

Bender RCMAs monitor DC and mixed AC/DC systems. The unique measuring principle can be used for protection if the DC system is grounded as shown below. In this case the negative pole of the DC power supply or the battery is tied into a chassis or the building ground. The active CT would be placed around both, the negative and positive conductor leading to the load. A DC leakage current will bypass the CT through ground. Its magnitude will be relayed to the RCMA device and an alarm will be triggered. The calculation is based on the same principles that were discussed in previous chapters. Remember, it is important to note that this technology works on AC, DC and mixed AC/DC circuits. For example: A single RCMA relay can protect a DC and an AC system at the same time. Imagine a DC control circuit and a 120VAC power wire going through the same CT. A standard GFR could only monitor the AC line. The RCMA will protect both.

V V

-

V+G= 24V V-G= 0V

Schematic: 24VDC grounded system with RCMA technology and single fault

+

IF = 1A

I- = 9A

I+= 10A

RCMA relay withactive transformer

Relays: RCMA470 (Motor protection) RCMA473 (Personnel)

Grounded Systems

12

Locating ground faults with portable equipment

Normally, it is not too difficult to locate a ground fault in a grounded system. As described before, the ground fault current is usually in the high amp range and will force the over current device or the ground fault relay to trip the faulty circuit. It can get tricky when there is only one ground fault relay installed at the power source, protecting a multitude of circuits down the road. A typical situation is the roof mounted air conditioner in an industrial plant. The AC unit faults to ground and trips out the main service entrance, because there was no branch protection provided. It will not be a simple task to find the culprit beneath 50 similar AC units on the roof. One solution to the problem is provided below:

Locating faults in disconnected systems (Off line search)

A typical means of checking for a ground fault in a disconnected system is the megger. A technician will connect a “meggering device” between the motor leads and ground (chassis) and inject a high voltage (normally 500V) into the motor circuit. The ground fault is indicated if the “megger current” finds a break through path to ground. Note: The megger only works on disconnected systems. Make sure that the test object is voltage free before applying the megger.


Ground

L1

L3

Schematic: Meggering a load

3Ph motor

Megger current

CB open!

500 Volts

Grounded Systems

13

Locating ground faults with a current clampLocating faults in life systems (On line search)

The current transformer based ground fault relay is basically nothing else but a very sensitive ampmeter. The typical clamp-on type ampmeter is used to measure load currents by simply clamping around a single conductor. The same ampmeter will read zero if the technician clamps around all conductors (Including the neutral, if there is one present) in a healthy system. This is the zero sequence principle we described earlier in the Grounded AC system 60 cycles section on page 8. A healthy system will reveal zero cur-rent but a ground fault current will show up immediately on the clamp. Please note: Do not incorporate the ground conductor when you put the clamp around the power wires. The ground fault will be chased by clamping around the conductors coming from the power supply first. From there we will work on the branches. From there we will start clamping around the individual loads. This is often described as “hunt-ing down” the problem.A typical clamp designed for measuring load currents will do the job if the ground fault exceeds a couple amps. A more sensitive clamp has to be chosen if the ground fault has a magnitude below 1 Amp. Below is the picture of a clamp which is actually capable of detecting faults below 10mA. Here comes the tricky part. Please be advised that using a clamp on a bolted fault is pretty much useless. The over current devices or GFI breakers will have tripped long before the handheld clamp can be employed. The meth-ods described above will only make sense at ground faults below breaker trip level. A disadvantage of measuring at lower amp levels will be the charging current of the system or the already present inherent system leakage. A rule of thumb in electrical installations is 1A leakage per 1000KVA. It will depend on the experience of the electrician to determine if he is chasing a charging current or a low level ground fault.

A simple example for explaining inherent system leakage:

The motor consists of windings, wire and insulation enclosed by a metal frame. A conductor separated from ground via insulation acts as a small capacitor. A piece of insulation between the conductor and the frame also has a certain resistance. The capacitance is usually extremely small and the resistance is in general in the Meg Ohm range. Nevertheless, the combined values can add up. The larger a power system is the larger the overall natural leakage will be.Imagine a motor in an industrial plant which leaks a minor fraction of current into ground (e.g. 1mA = 1/1000 of an Amp). That does not sound like very much and the decision is made to employ a ground fault relay with a 10mA set point. The GFR trips immediately because it was overlooked that there are 15 similar motors connected to this branch. 15 x 1 mA = 15mA leakage already present in the system. This does not even account for the capacitive leakage of cabling or for drive components with their built-in EMI filter circuitry.

hot

neutralDevice: RCT3283Accessory: Large clamp RCT9131

Grounded Systems

14

Locating ground faults with fixed equipment

Online system

Assume that there is a 208VAC three phase system with 120 sub branches installed in a facility. The supervisor wants to know at all times if the system is in good shape or if there is a problem. A problem has to be indicated immediately, the source has to be identified and fixed. The zero sequence technology described in earlier chapters can help here as well. A fixed installed ground fault scanning system can be employed to monitor unlimited numbers of branches 24/7. In this case, a CT will be placed around every single main and sub branch. The CTs will be connected to 12 channel evaluators which themselves are wired into a central processing and display unit. The ground fault will flow from the source into the faulty branch, into the faulty load and from the ground back to the source. The CTs in this path will recognize the current and alert the respective evaluator. The central processing unit will display the amount of fault current and the fault location. Remote alarms can be sound and even the alert via the internet is no problem nowadays. Fixed ground fault location systems are tremendously beneficial when down- time is not an option. Please note that the CTs are measuring the full amount of ground fault current and are designed to detect leaks in an early stage. Fast developing ground faults will most likely trip the over cur-rent device before the RCMS will locate the fault. The RCMS in a solidly grounded system will perform at its best when the fault level stays below the branch breaker trip levels. Above these values, the branch protection will kick in.

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TGH 1270E / 03.97

Product Description

Basic system A basic RCMS470 system consists of an RCMS470-12 evaluator and one to twelvetransformers. A system of this kind offers the following facilities.

• Monitoring twelve transformer outgoing circuits within an TN or TT systemwith a common response value for all channels.

• Central monitoring on the RCMS470-12 by means of alarm LEDs andalarm relays.

A1 A2 K1 K2 K3 K4 K5 K6 K7 K8 K9

A B R1 R2 l K12 K11 K10 11 12 14

10

RS485TEST

RESETFAULTALARMON A4 A3 A2 A1 A0

K12K11K10K9K8K7K6K5K4K3K2K1RCMS470

I∆n

ADDRESS

MONITOR

SLAVE

MASTERY D2 D1 D0 R/-

MASTER:D2 D1 D0 Y/mA

0

0

0

1

1

1

1 1 1 1 A

1 0 500

0 1 300

0 0 100

1 1 50

1 0 30

0 1 10

AC TN-SystemL 1

N

For supply voltage referto nameplate

F X

W

PE

W W

KeyExample of a basic RCMS470 system with an RCMS470 and three W1-S35measuring current transformers in a single-phase AC network (TN or TT system).

W: Measuring current transformer. In this example three W1-S35 transformersare used. Up to a maximum of twelve measuring current transformers canbe connected.

F: Protective device for supply voltage to protect against short-circuit. A 6 Afuse is recommended.

X: Terminal strip for common connection of the measuring current transformers.The maximum distance between terminal l for the RCMS470-12 and terminalstrip X is 25 cm where the cross-section is 2.5 mm2 and 15 cm where the cross-section is 1.5 mm2.

Picture out of RCMS470

Device: RCMS470 Scanning system

Resistance Grounded

15

Basic power systems - Resistance Grounded

Resistance grounded systems are created if the neutral is tied to ground via NGR (Neutral Grounding Resistor). Typically these NGRs can be found in three phase Y configured mining power systems. Industrial plants employ these systems on occasion as well. A main advantage of the resistance grounded system is that the resistor limits the amount of current available to the fault. The operation may be continued until the electrician locates the fault and fixes the problem.

The ground fault

A ground fault current in a resistance grounded system will be limited in its magnitude. This is the main difference when compared to the solidly grounded system we encountered before. The ground fault magnitude depends on the system voltage, the resistance of the ground fault causing part and the resistance of the current limiting resistor. Typical limits are 5, 10, 15 or 20 Amps. The resistor should be rated for continuous use under full load if there is no protection device on this circuit.

The resistance grounded system also allows for selective ground fault tripping which is achieved through adjustable time delays on multiple relays.


IF Fault CurrentV3G Voltage between faulted phase and groundRGF Resistance value at shorted pointRGR Resistance of ground pathRNG Resistance of neutral grounding resistor(Please note: The resistances were randomly chosen)

IF = 277V/(0.1Ohm + 0.2 Ohm + 55Ohm) = 5A

The calculation shows that a theoretical fault current will not be as devastating as in a solidly grounded system if a dead short occurs. The maximum fault current will be limited, therefore vital machinery can be kept running until the process is finished. Of course, problems should be fixed ASAP.


Ground

L1

L2

L3

Schematic: The resistance grounded system

3Ph motor

GF = i.e. Dead shortResistance = 0.1 Ohm

Fault current path via groundResistance assumed 0.2 Ohms

NG pathResistance 55 Ohms

Resistance Grounded


Ground

L1

L2

L3

Schematic: The resistance grounded system with GFGC ground fault ground check relay

3Ph motor

GFGC Ground fault Ground check relay

1

23

Ground pilot wire

Ground Checkterminationdevice

NGR

GF

1.1

16

Resistance grounded AC systems

GFGC Ground Fault Ground Check

The resistance grounded system could employ a typical 60 cycle ground fault relay to ensure electrical safety. However, resistance grounded systems in modern power applications require more than just the ground fault part. A perfect ground connection has to be established to ensure that dangerous frame volt-ages will be clamped effectively to ground. A combination GFGC (ground fault - ground check) monitor will solve the issue. This relay will measure the residual current in the respective circuit or branch of the system by means of a CT. For that purpose, all active conductors (phases + neutral) are to be passed through the CT (1).

If the ground fault current (1.1) exceeds the response value, the “Alarm Ground Fault” LED lights and the alarm relay switches. The alarm contact can be delayed by a selectable time. The alarm remains stored until the RESET button is pressed.The GC relay function will monitor the impedance of the grounding conductor. For that purpose, the relay superimposes a voltage of 12V between the grounding conductor and the ground check wire. A termination device between both conductors at the other end of the cable has to be installed. By evaluating the voltage drop at this termination device, the GC part recognizes series resistance faults (cable high-resistance or open) (3) or cross resistance faults (short circuit) (2) of the cable.The alarm relay trips immediately if one of the above occurs.

Device: RC48C

Resistance Grounded

17

Resistance grounded AC systems

Ground Fault - NGR Monitor

The resistance grounded system will limit possible fault currents by means of the NGR (Neutral Grounding Resis-tor). A typical CT (1) based ground fault relay can be used to sense the ground fault current. What would hap-pen if the current limiting resistor is destroyed or looses one of its connections (2) ? The answer is simple. The resistance would become infinite and the former resistance grounded system would become floating. As learned before, the typical GF relay is not effective on floating systems and is therefore now useless.A common practice is to employ a potential transformer across the neutral grounding resistor. The potential trans-former will indicate a voltage rise across the transformer caused by a ground fault current. However, the voltage rise will occur disregarding if the resistor is online or not. Therefore, the potential transformer makes sense as a ground fault protective device, but does not provide an integrity check for the resistor.

Please review the schematic below. The Bender RC48N series will provide a solution. If the GF current (1.1) exceeds the response value, the “Alarm Ground Fault” LED lights and the alarm relay switches. The NGR part monitors the resistance of the neutral grounding resistor, connections through the transformer, and the connections to ground (2). It also monitors the voltage drop on the neutral grounding resistor via a coupling device. An alarm is indicated when the ground-fault current or the neutral ground voltage exceed the set point. The RC 48N can be used to backup or evaluate the potential transformer.


Ground

L1

L2

L3

Schematic: The resistance grounded system with GF NGR ground fault relay NGR monitor

3Ph motor

GF NGR Ground fault relayNGR monitor

1

2

NGRNGR coupler

1.1

GF

Device: RC48N

Resistance Grounded

18

Substation courtesy of Metalec

RC 48C

The RC48C is built into the substation shown above. It will trip out a contactor if one of the fol-lowing conditions should occur:A) Ground faultB) Ground wire resistance increasedC) Ground wire cross fault

Locating ground faults

The resistance grounded system is subject to the same techniques as described in the grounded sys-tem. The only difference is that the possible fault current is limited to 5, 10 or 20 Amps. The same zero sequence CT based technologies apply.> PLEASE REVIEW THE PAGES 13 & 14 FAULT LOCATION IN GROUNDED SYSTEMS.

Application picture

Ungrounded Systems

19

Basic power systems - Ungrounded (Floating)

Floating systems are derived from a power source where there is virtually no connection to ground. 480VAC delta configured transformers are a typical supply for a floating system. Some deltas in the mining industry can be found in hoists. 480VAC deltas are also in wide spread use to supply 1000Amp - 2000Amp main feeder circuits in general industrial applications. Floating systems are often used in areas where a sudden shut down must not occur. Examples are Intensive care units (ICUs) in hospitals, signal circuits, and emergency back-up systems.

The ground fault

The magnitude of a ground fault current in an ungrounded system is very small. It depends on the sys-tem voltage, the resistance of the ground fault causing part and the system capacitances.

Example: If a grounded object with low resistance touches a live conductor, the resulting current flow will be negligible. The ground fault loop will be incomplete because the return path to the source is missing. It is important to note that the systems capacitance will provide some paths for capacitive currents to flow. The resulting current is also known as charging current. Never assume that it is safe to touch a bare conductor in a floating system. A capacitive jolt will be extremely hazardous. The example below reflects resistive values only. The capacitance, though important, was not added to the equation.


IF Fault CurrentV3G Voltage between faulted phase and groundRGF Resistance value at shorted pointRGR Resistance of ground pathRNG Resistance between Neutral and Ground (In this case: Mega-Ohms through air)(Please note: The resistances were randomly chosen)

IF = 277V/(0.1Ohm + 0.2 Ohm + 1MegOhm) = 0.00027A = 0.27 mA


Ground

L1

L2

L3

Schematic: The ungrounded system with ground fault path

C1 C2 C3

Leakage capacitance

GF = i.e. Dead short

Ungrounded Systems

20

Insulation Monitoring Device IMD

Ungrounded systems will not produce the amount of fault current needed to trip a common GFR. The IMD is the device of choice for the protection of floating systems. IMDs come in two styles: A) Passive and B) Active devices.

Passive IMDThe most known passive device is most likely the three light bulb system in industrial 480V delta systems. Three lamps are connected on their secondary side together and from there to ground (Star or Y con-figuration). Each lamp is then connected to the respective phases L1, L2 and L3. In a healthy system, all three lights will burn with the same intensity. In case of a ground fault, the faulted phase will assume a value close to ground potential. The respective light will dim, while the other two will brighten up. The light bulb system often does not offer additional trip indicators for remote alarms. It also needs to experi-ence a serious fault condition before people are becoming aware that something is going wrong. Even worse, symmetrical ground faults (A balanced fault on all three phases) will not be detected.


V1= 277V V3= 277VV2= 277V

Ground

L1

L2

L3

Schematic: Three light bulb system under no fault condition

C1 C2 C3

Leakage capacitance

3Ph motor


Supply side 480VAC

V1= 480V V3= 0VV2= 480V

Ground

L1

L2

L3

Schematic: Three light bulb system under single fault condition

Ungrounded Systems

21

Insulation Monitoring Device IMD

Passive IMDs Ungrounded DC systems are often equipped with a passive measuring device. The “balance” method is the most commonly known method to protect a floating DC bus. Two voltmeters are connected between the positive, negative lead and ground. The voltmeter acts like a voltage divider. Each voltmeter should indicate about 6V in a 12V system. A ground fault will clamp one of the voltmeters closer to ground and it will indicate a value closer to zero. The other voltmeter will see an increase and move up closer to 12V. The magnitude of this shift will depend on the magnitude of the ground fault. As with all passive systems, balanced ground faults will not be detected with this technology. These devices also do not provide early warning or trending capabilities.

V V

-

Battery system 125VDC

+

V

V

V V1N= 125V

V2N= 125V

V12= 250V

N

V1G= 125V V2G= 125V

125V loads 250V loads

Schematic: 125/250VDC system voltage measurements double fault condition

GF = resistance breakdown to 1000Ohms on two legs i.e. cable under water(Balanced fault situation)

V V

-


+

V

V

V V1N= 125V

V2N= 125V

V12= 250V

N

V1G= 125V V2G= 125V


Schematic: 125/250VDC system voltage measurements no fault condition

V V

-


+

V

V

V V1N= 125V

V2N= 125V

V12= 250V

N

V1G= 250V V2G= 0V


Schematic: 125/250VDC system voltage measurements single fault condition


1) Passive IMD -> Review the UG140P series

Ungrounded Systems

22

Active IMDs

The active IMD is considered to be an online megger. It will be connected via pilot wires between the system and ground. A constant measuring signal will be sent from the IMD into the power wires. It will spread out evenly into the secondary side of the supply transformer and the attached loads. If this signal finds a break through path to ground, it will take this path of least resistance and return to the monitor. The IMDs internal circuitry will process the signal and trip a set of indicators when the set point is in-creased. IMDs measure in Ohms (Resistance) and not in Amps (Current). A ground fault will be indicated as “insulation breakdown”.

Great insulation = healthy system = multiple kilo ohms or mega ohmsLow insulation = ground fault = less than one kilo ohm or low ohm range

A power system’s overall resistance depends on the number of loads, the type of insulation used, the age of the installation, the environmental conditions etc. A typical question when it comes to floating deltas is always: “Where should my set or trip level be?” The typical “ball park” figure for industrial applications is 100 Ohms per volt.Example: A monitor in a 480V delta system would be set to trip at 480 x 100 = 48 kilo Ohms. Please be advised that this figure cannot be used for all situations. Example: A customer has meggered a mo-tor and figures that his system must be at around 1 Meg Ohm insulation. The insulation monitor keeps alarming and indicates lower levels than previously assumed. The answer is simple: It was forgotten to take into consideration that there are 10 of these motors connected to the same system. The IMD will measure and indicate the OVERALL resistance of the system. Here we are dealing with 10 parallel resis-tances of 1 Meg Ohm each. The overall resistance will drop to less than 100000 Ohms in this case.

3Ph motor


Supply side 480VAC

V1= 480V V3= 0VV2= 480V

Ground

L1

L2

L3

Schematic: Ungrounded AC system with Bender IMD

AMP measuring pulse

Bender IMD

1) Single phase AC systems -> Review the IR140Y series2) Single phase AC, DC or mixed systems -> Review the IR145Y series3) Three phase AC systems -> Review the IR470Y series4) Three phase AC, DC or mixed systems -> Review the IR475Y, IRDH275 and IRDH375 series5) Voltages above 500VDC,690VAC -> Review the AGH couplers

Ungrounded Systems

23

Active IMDs

An active IMD is the preferred choice for ungrounded DC systems as well. As in floating AC systems, a DC IMD will be connected via pilot wires between the live (or dead) system and ground. A constant mea-suring signal will be sent from the IMD into the power wires, from there it spreads out evenly into the sec-ondary side of the supply (E.G. battery) and the attached loads. If this signal finds a break through path to ground, it will take this path of least resistance and return to the monitor. The IMDs internal circuitry will process the signal and trip a set of indicators when the set point is increased.

V V

-


+

V

V

V V1N= 125V

V2N= 125V

V12= 250V

N

V1G= 125V V2G= 125V


Schematic: 125/250VDC system with Bender IMD technology

GF = resistance breakdown to 1000 Ohms on two legs i.e. cable under water(Balanced fault situation) AMP measuring

pulse

Bender IMD

1) DC system (Active IMD) -> Review the IR145Y, IR475Y, IRDH275 and IRDH375 series

Ungrounded Systems

24

Ungrounded systems - Locating ground faults with portable equipment

Locating faults in ungrounded deltas is different from doing so in a grounded Y. A typical leakage current clamp will not perform. We learned in earlier chapters that even a dead short to ground will not create hazardous currents in floating systems. If there is no GF current flow then there will be nothing available to be picked up. The solution is to send a low level artificial signal via pulse generator into the faulted system. The signal will be impressed between the power wires and ground. Naturally, it will follow the ground fault path into ground and return to the pulse generator. This signal can be traced with a hand held probe which appears to be similar to the current probe discussed in previous chapters. Neverthe-less, it has to be emphasized that this is a specially designed clamp which will trace the pulse and not ground fault currents. Both the clamp and pulse generator will work in unison. One without the other will be useless in a floating system.

33

IT-System

L1

L2Ground

PULSE GENERATORBENDER PGH 185 PULSE

TRACER

EDS3065 in floating deltaEDS3365 Ground fault location kit for AC/DC systems below 300V

EDS3065 Ground fault location kit for AC/DC systems above 300V

Ungrounded Systems

Ground fault location in a 480V delta fed system

A manufacturing plant encountered a decrease in insulation value. The system broke down from 45Kilo Ohms to less than 5Kilo Ohms. Shutting off breakers did not reveal any improvement. The EDS3065 fault locator was chosen for the task since we were dealing with a 480V 3Ph system. (2000A main bus, 42 branches)

Picture #1 The PGH pulse generator is connected via pilot wires. The injected signal can be seen on the hand held EDS165 evaluator device. There are 12 branches in this panel. Each one has to be checked for the pulse. That takes approx. 30 seconds each. We locate the pulse in branch 2F7.

Picture # 2Branch 2F7 leads us further into the production area. The pulse is still strong. It is amazing how accurate the device is, considering the fact that we are only sending 25mA into the system. We have now arrived at a sub panel leading to various machines and water cooled transformers.

Picture # 3The evaluator shows 15mA going through these water cooled transformers into ground. The transformer specs and the local electri-cians verify that this is a normal situation. But there are still 10mA vanishing beyond this point. Lets continue the hunt.

Picture # 4The culprit is found. The remaining 10mA led us to this transformer which was well hidden inside a metal bending machine. Obviously, there is a need for a replacement part.

25

Ungrounded Systems

26

Ungrounded systems - Locating ground faults with fixed equipmentThe ground fault location system can be installed as a fixed installation if 24/7 monitoring should be re-quired. A complete ground fault location - detection scheme for a floating system incorporates:

1) IMD Insulation Monitoring device Function: Detects the fault and alarms when the set point has been reached. IMDs were discussed on pages 23 & 24.

2) Pulse generator Function: Sends the trace pulse into the power wires once a fault is detected. (Please review page 25 for more background information on the test pulse issue)

3) CTs Current Transformers Function: Specially designed for sensing the trace pulse and sending information back to the evaluator.

4) Evaluators Function: Information (Fault) gathering from the CTs and indicating the faulty branch.

5) Central Control Unit Function: Gathers the information from the evaluators and displays alarms.

The Bender EDS ground fault location - detection system is an excellent tool for the maintenance person-nel in a large facility with extensive wiring. Faults will be located automatically during normal business operation. No shutdown required. No handheld tracing and/or accessing panels required. The beauty of this system lies in its non invasive operation. If the case study on page 25 would have been equipped with a fixed GF location system, the faulty branch and its connected machine would have been identified during the first 120 seconds after the first alarm emerged. The ungrounded system is only safe for its user as long as the occurring faults are immediately traced down and eliminated. If that is not the case, then the second ground will follow sooner or later and short circuit the system.

∆

1 ALARM 2

TESTRESET

Ω

1,2 & 5combined in one unit

4

3

3

3

2

Ungrounded Systems

EDS470 Ground Fault Location SystemTwo insulation monitors connected to a series EDS470-12 evaluators. This is a smaller system ca-pable of evaluating 2 x 12 circuits.

EDS473 Ground Fault Location SystemOne insulation monitor is connected to a multiple evalu-ator devices. A total of 82 circuits is being monitored for ground faults. The customer decided to add a data logger for trending purposes (Top).

27

By the way, we are often asked how large our devices are. 95% of the Bender relay line is relatively small and has been designed to be mounted into control cabinets or panels.

See for yourself:

Off-Line Monitoring

28

Off-line monitoring with Bender IR … devices

Off-line monitoring addresses the monitoring of idle consumers, to avoid activation in the presence of a ground fault.

The Off-line monitor is basically a fixed installed control relay which duplicates the function of a technician who “high- pots” or “meggers” a load to check for ground faults or insulation break downs. These loads can be de-watering pumps, fire pumps, motors or any other electrically fed piece of equipment which does not operate on a continuous basis. In many installations, even continuously running equipment will be shut down in regular intervals to perform a high voltage line to ground test. The megger task involves disconnecting the load from the power system. Then the high voltage device or so called megger will be connected to the motor leads and ground. The voltage will be superimposed onto the wires and windings for a brief moment during which the technician looks out for a break through indication. The complete task will take up a considerable amount of time. The job sites are often in remote areas, wiring and disconnecting tasks have to be performed and safety regulations have to be fulfilled. Amazingly enough, it is widely unknown that this manual task can be fully automated by using a low cost relay. Off-line IMD’s (Insulation Monitoring Devices) superimpose a DC measuring signal to the system being monitored in various applications across the nation.

The relay will alarm if the superimposed signal finds a break through path to ground. The available Bender off-line IMD’s are shown in table 5.1. The table also indicates the measuring voltage (max.) of the individual IMD which will be superimposed between the power wires and ground.

3Ph motor on stand-by


Supply side 480VAC

Ground

L1

L2

L3

Schematic: Offline monitor under fault condition

Contactor main (Open)

Contactor aux. (Closed)

IT = Test current

Water intrusion

Offline monitor

Off-Line Monitoring

29

The Off-line IMD’s and their measuring voltages

Bender Type Measuring Voltage (max.)

IREH1520 500V dc

IR470LY2-60.. 40V dc

IREH470Y2-6.. 20V dc

IREH470Y2-60.. 40V dc

In recent investigations, the impact of the measuring voltage on the accuracy of the measurement was tested. The results showed only slight variations of the accuracy with a decreasing measuring voltage.

Still, various clients prefer to use the Bender Type IREH1520 with a measuring voltage of 500V dc which is equivalent to the one used in common “Megger” testers. Mining customers generally lean more towards the low voltage off-line monitoring devices. There are various standards addressing the off-line issue in various applications across the nation.

Standards for Off-line monitors

One of them is the American standard, ASTM F1134-88, which describes the monitoring of idle consumers in shipboard applications. In this standard the max. measuring voltage allowed is 24V dc. The IEC standard, IEC61557-8, (Title: “Insulation Monitoring Devices for ungrounded AC Systems, for ungrounded AC systems with connected DC circuits and for stand alone DC systems”) describes the IMD’s in general, limiting the measuring voltage to 120V (peak value). The diagram below shows an example for an off-line IMD Bender Series IREH1520 and IREH470Y2.. . Those IMD’s are connected to the system being monitored via a pilot wire. An auxiliary contact will be used to initialize the IREH relay when the motor goes off-line 8).

8) In applications with a grounded supply, the contactor needs to interrupt all power lines including the neutral (if available).

Off-line IMDs often pay themselves off during their first year of operation. A typical 100HP pump motor whose seals have failed can easily be put back into operation by repairing the seals and drying out the stator windings. This will work only if an off-line monitor prevents a “wet start-up”. Rewinding the stator instead will cost the operator thousands of dollars and unnecessary down time.

Table 5.1

30

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/DC

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Typ

eIR

470LY

IR475LY

RC

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RC

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RC

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RC

48 N

HW

135

RM

475

IRE

H470Y

Resp

on

se v

alu

e1..2

00kO

hm

2...5

00kO

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30...3

00m

A30m

A...3

AI

0,1

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I 0

,1...1

0A

I 1

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50...1

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hm

0,1...2M/0,5..10M

No

tes

1 A

larm

LE

D-B

arg

raph

2 A

larm

sLE

D-B

arg

raph

exte

rnal C

Texte

rnal C

TGround continuity and shorted ground m

onitoringvia ground check wire

voltage and current monitoring of

groundingresistor

grounding conductor for low and high resistance and shorted

Ground continuity and shorted ground m

onitoring via ground check wire

2 A

larm

s

Typ

eIR

DH

275

RC

M465Y

RC

MA

475LY

Resp

on

se v

alu

e1k...1

0M

Ohm

30...3

00m

A30...5

00m

A

No

tes

2 A

larm

sLC

-Disp

lay

inte

rnal C

T 1

8m

min

tern

al C

T

Typ

eIR

DH

375

RC

M 4

70LY

Resp

on

se v

alu

e1k...1

0M

Ohm

10m

A...1

0A

No

tes

2 A

larm

sLC

-Disp

lay

exte

rnal C

T

Typ

eIR

DH

575

RC

M 4

75LY

Resp

on

se v

alu

e1k...1

0M

Ohm

10m

A...1

0A

No

tes

for ground fault locationin

tern

al C

T

Typ

eW

-, WR

-, WS

- serie

sW

...A se

ries

Resp

on

se v

alu

eN

ote

sstandard, split-core, rectangular

35...2

10m

m

Tra

iling ca

ble

s up to

5kV

Min

ing P

roce

ssing

Pow

er g

enera

tion

Pow

er d

istributio

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Min

ing P

roce

ssing

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enera

tion

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AC

Insu

latio

nM

on

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ungro

unded

Gro

un

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rela

y

solid

ly gro

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Gro

un

d fa

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rela

y &

Gro

un

d

co

ntin

uity

mo

nito

r CS

A M

421-0

0

solid

ly gro

unded a

nd re

sistance

gro

unded

pa

ge

1 o

f 1se

lectio

n g

uid

e (2

) for re

lays.xls

Selection Guide for

Ground Fault R

elays

* Data sheets available at:

w

ww

.bender.org-Products

BENDER Inc.700 Fox ChaseCoatesville, PA 19320Tel. (800) 356 - 4266Fax. (610) 383 - 7100E-mail: [email protected]

electrical safety in grounded, resistance grounded and ug systems

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