notes on tripping relays

Siemens Protection Devices Ltd., PO Box No. 8, Hebburn, Tyne & Wear, NE31 1TZ, England

Technical application notes and selection of Tripping Relays: Introduction. DC relays associated with a tripping function (Trip Relays) are generally used within the protection scheme to either:

1. Co-ordinate the tripping outputs of several protection devices 2. Multiply the number of available contacts 3. Increase the contract rating 4. Act as a Sacrificial relay between the protection and circuit breaker

Type 7PG15/TR relays are a range of multi-contact attracted armature all or nothing relays designed to both IEC 255-5 and to BS142. A wide range of models are available to meet the requirements of the electric supply industry. They have a high speed, positive action and are of robust design for a long, reliable service life. Figure 1 – Typical DC Tripping Scheme – Single pole switched.

N.B. The circuit shown is typical for a latched trip relay. Reset connections not shown.


Figure 2 – Typical DC Tripping Scheme - Double pole switched.

N.B. The circuit shown is typical for a latched trip relay. Reset connections not shown. Low Burden TR1 series. Type TR1 relays are suitable for application for tripping and auxiliary duties where immunity to capacitance discharge is not required or the circuit is designed in such a way the relay operation is not effected by such currents. Low burden relays are generally installed where the wiring from the initiating contact is local to the trip relay and therefore not subject to capacitive discharge currents that could cause incorrect operation. Alternatively the scheme can be designed using an initiating contact in both the positive and negative circuit (double pole switched), figure 2 of the trip relay coil. These relays are not intended for use with current operated series follower relays. High Burden TR2 series. Type TR2 relays have a high burden (relative to TR1) and thus have immunity to capacitance discharge currents which can arise due to earth faults on DC battery systems. A high burden also provides reliable operation of current operated series flag or repeat elements (such as type FR).


Application. Trip Relays are required to meet a number of criteria for correct, reliable operation and to exhibit immunity to both induced voltages and maloperation when earth faults are present within the DC supply system. The type TR1xx / 7PG15 relay is designed to meet or exceed the requirements of the UK standard EATS 48-4 category EB1 . The type TR2xx / 7PG15 relay is designed to meet or exceed the requirements of the UK standard EATS 48-4 category EB2. This standard defines the required performance to be met by relays when used as tripping relays. Voltage levels. The specified minimum operating voltage for EATS 48-4 category EB1 and EB2 relays is 50% of the Battery Nominal Working Voltage. This is in line with associated standards for circuit breaker trip coils, which traditionally must also operate at 50% of nominal volts. The 50% voltage level has been chosen to ensure correct operation of the tripping systems even when there is a loss of charger supply for considerable periods. As the relay is guaranteed to operate at 50%of nominal volts, it is designed and set to operate at an even lower level. This guarantees correct operation at 50 % voltage, and allows for manufacturing tolerance on devices. Consequently it will be found that relays will operate below 50% of rated voltage, this is normal and correct.


Earth Leakage currents. EATS 48-4 specifies a minimum operating current below which a relay must not operate. This is designed to provide immunity to Earth Leakage currents expected in service conditions. Category EB1: For 30 & 48V DC relays, the specified minimum operating current is 10mA. For 125V DC relays, the specified minimum operating current is 25mA. Category EB2: For 30 & 48V DC relays, the specified minimum operating current is 20mA. For 125V DC relays, the specified minimum operating current is 50mA. Example: Consider the application of an earth fault to the wiring between a protection device output contact and the trip relay coil: As the 125V battery is centre point earthed, the maximum applied voltage will be 62.5V. Typically a battery earthing resistor is of 6000 Ohm (or greater) resistance. In the worst case (neglecting the effect of wiring and relay coil resistance), the maximum earth fault current which can flow is (62.5/6000) 10.4 mA. This exceeds the minimum operating current of an EB1 class relay, which would tend to maloperate. However, this is less than 21% of the minimum current required to operate an EB2 class relay. In this case an EB2 class relay will not operate for an earth fault, an EB1 class relay will operate. Capacitive Discharge currents. In general, D.C. relays with low minimum operating current and high operating speeds are liable to be operated by capacitance currents. Capacitance currents may flow through the relay coil circuit if, for example, an earth fault occurs on the D.C. circuits associated with the relay. Relays are less liable to maloperation if they:

1) Have both sides of their coils switched. 2) Have no external wiring associated with them. For example, where the relays

are directly mounted on metal-enclosed switchgear or where all the wiring connected to the coils of the devices is confined to within the relay room and is not taken directly to any external device.


Unless these conditions apply or additional precautions are taken, the relays should be regarded as being at risk of a maloperation due to a capacitive discharge resulting from an earth fault. Under healthy conditions, the supply wiring will carry a capacitive charge to earth, maintained by the battery voltage. EATS 48-4 specifies a capacitance discharge test to ensure that relays remain stable in the event of an earth fault causing the energy (stored in the cable / earth capacitance ) to discharge through the relay coil and associated components. This test requires an EB2 relay to withstand the discharge of a capacitor (10 microfarad, charged to 150V DC) without operating. This value of capacitance is greater than that seen in real applications; the energy applied to the relay under test is therefore greater than that experienced in use. TR2xx / 7PG15 series relays have all been tested as above. No operation took place, therefore they have passed this test. Where relays are at risk of a maloperation due to a capacitive discharge, category EB2 / High Burden devices provide the highest immunity and should be used.

Trip relay timing measurement

Background: SPDL Tripping Relays operate in a maximum time of 10ms. The time being measured from relay initiation on full supply voltage until first closure of the tripping contact. Fundamentally, there are two measurements which may be taken from such a relay: "First Touch" - the time taken for the first make of the output contacts of the relay. "Fully Home" - the time taken for the relay to operate, close its output contacts fully and all related contact wipe/bounce to cease. SPDL published operating times are "First Touch" times. The use of “First Touch” time is specified by IEC 61810, BS142 & DIN41215, which are the applicable standards for these devices. It has been proposed that the Trip Relay should be “Fully Home” within 10ms; it was suggested that the contact wipe/bounce seen on these devices can cause a significant delay to Circuit Breaker tripping. This is not the case; in practice, much of the measured contact bounce is created by varying resistance as the contacts wipe against each other. This characteristic of


contact operation is not detrimental to contact performance in normal operation, actual contact separation being limited to extremely small transients. CB tripping circuits will operate from voltages over 24V DC and the contacts initiate relays/coils with sufficient burden and inductance that, once the circuit has made, any minor contact variations do not interrupt the circuit. Current continues to flow whilst the contacts wipe/bounce against each other. This further supports the standard specification of “first touch” as the correct point for timing. Evaluation testing suggests the maximum delay to a tripping circuit, due to contact wipe/bounce will be in the order of 2ms. Testing: Many commercially available site test sets/timers do not perform a true "First Touch" time measurement. The timer response to contact wipe/bounce is usually not published or defined, and the extent to which the results are affected is not readily predicted. Therefore the times recorded by this equipment will not be "First Touch" or "Fully Home", but an undefined measure of the timer’s reaction to the relay contact characteristics. For trip relay timing a true “first touch” timer must be used, otherwise acceptable test limits must be revised to allow for the incorrect readings of the test equipment. In modern test equipment, the voltages used are often not sufficient to provide a “wetting” voltage that would maintain the circuit whilst the contacts are wiping. SPDL recommend a minimum of 24V DC as a wetting voltage, based upon the contact materials used. When used on otherwise unburdened contacts, commercial timing test sets are likely to measure the operating times of all trip relays as slower than design, but the characteristics of contact make & wipe on SPDL TR relays makes them prone to the measured time being outside of acceptable limits. Whilst this is indicative of how electronic circuits might see a closing contact, it is not representative of how tripping circuits operate. The true tripping time can only be measured when contacts are performing tripping duty.


Contact Resistance Measurement. In order to correctly measure the contact resistance of any electromechanical relay the following method must be adopted: An external power supply of at least 24 Volts and a series resistor should be used to pass a measured current through each contact pair, the resistance is to be calculated from the applied voltage and current. A multimeter cannot be relied upon to indicate the contact state correctly due to the minimum wetting voltage required by the contact tip material. As the multimeter battery is 9 volts or less, any measured resistance values will be unreliable, incorrect and too high in value. Conclusion. TR2xx / 7PG15 relays are compliant with EATS 48-4, and are immune to maloperation due to earth leakage currents. The low pickup voltage does not make the relay prone to maloperation by earth fault current. The limiting factors are the battery earthing resistor and the minimum operating current. In practice the immunity to maloperation is further increased by the resistance of the relay coil, site wiring and the use of a higher value battery earthing resistor. Maloperation due to capacitance discharge is not possible as the TR2xx / 7PG15 devices have all passed the required capacitance discharge test at energy levels greater than those seen in practice. Relay operating times, when measured correctly, will be less than 10ms. The correct test equipment and test method must be used as discussed above.

M. Holden PLM & Senior Applications Engineer, Siemens Protection Devices Ltd. 09-01-2013

notes on tripping relays

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