earthing / grounding issues

Joseph Ong Aug 2005 Company Confidential 1

Earthing / Grounding Issues

Reasons for Earthing and Bonding• Detect fault current and fast removal of

power source at fault• Prevent potential differences which may

cause electrocution or sparks• Minimise the effect of lightning strikes• Prevent build-up of ESD• Minimise the effect of electrical interferences• Meet Explosion-Proof apparatus safety

requirementsReference: MTL website


Earthing / Grounding Issues

Ground / earth resistance and resistivity should be measured when:

• Installing new ground systems and electrical equipment

• Periodically testing of ground and lightning protection rods

• Prior to design of ground protection systems


Earth Testing Methods

• Fall of Potential – 3 and 4 Pole Testing

• Resistivity

• Selective Testing

• Stakeless Testing


Earth Testing Methods (1)

Fall of Potential – Three and Four Pole Testing


Earth Testing Methods (1)Fall of Potential – Three and Four Pole Testing

RH RS RE

V

25m 25m I


Earth Testing Methods (1)Fall of Potential – Creating the ‘S’ Curve

• The positioning of temporary electrode for the test is a critical issue

• The distance between electrodes is dependant on the ‘pool of potential’ of the earth system under test

Dista nc e (m )

• Distances of temporary electrode should be varied to plot values as an ‘S’ curve


Earth Testing Methods (2)Resistivity Measurement

From the indicated resistance value RE, the soil resistivity calculates according to the equation :

E = 2 . a . R E

E ...... mean value of soil resistivity (.m)RE ...... measured resistance (a ...... probe distance (m)


Principle: Wenner – MethodPrinciple: Wenner – MethodMeasurement of specific earth resistance Measurement of specific earth resistance

The measuring method according to Wenner determines the soil resistivity down to a depth of approx. the distance "a" between two earth spikes. By increasing "a", deeper layers can be measured and checked for homogeneity. By changing "a" several times, a profile can be measured from which a suitable earth electrode can be determined. According to the depth to be measured, "a" is selected between 2 m and 30 m. This procedure results in curves:



Curve 1:As E decreases only deeper down, a deep earth electrode is advisable

Curve 2:As E decreases only down to point A, an increase in the depth deeper than A does not improve the values.

Curve 3:With increasing depth E is not decreasing: a strip conductor electrode is advisable.

Resistivity Measurement


Earth Testing Methods (3)Selective Measurement Method


Earth Testing Methods (3)Selective Measurement Method

RH RS RE

V

25m 25m

A

I



Principle: Selective earth measurementsPrinciple: Selective earth measurements

Advantage: Distinct earth resistances within meshed ground systems can be measured without disconnection. Only clamp current Im is used for calculation of RE.

Important: The minimum clamp current is 0.5mA – if not, an error message is displayed.

R Display = R E3 !!

Im

MeasuredvoltageMeasured current IM

Probe

Auxiliaryearthelectrode


Earth Testing Methods (3)Selective Measurement Method on High Tension Towers


Earth Testing Methods (3)Measurements on high tension towers

• Applicable for nearly all metal constructions!

• Example: Tower with four stands.

• Important: Do not move the current injection point (E) during test!

• All stands are tested in sequence with the split core transformer.

ring earthelectrode

split core transformer



RDisplay= U / IE = RE

Connectionto equi-potentialbus bar Equi Potential bus bar

Water pipe Earth electrode

RE Water pipe

IE Water pipe Isum = IE + IE Water pipe

Probe - (S)(voltage reference)

Auxiliary earth - (H)(injection of test current)

Application: Function RApplication: Function REE selective selective

Only the current from the current clamp is used for calculation of RE.

The precise value of RE is on display!

RE

sum

SATURN GEO X, SATURN GEO plus


Earth Testing Methods (4)Stakeless Measurement Method



Assume that there are Rn resistances in parallel (e.g. in meshed ground systems, telecom, electric power distribution systems) earth connections are inter-connected.

Thus Rn<<Rx is valid. Rn is negligible and the measuring loop resistance is mainly Rx.

In this case the value on display of the In this case the value on display of the

ground loop tester is exactly Rx.ground loop tester is exactly Rx.

Current amplifier

Voltage

source

II

RxRn

U

Stakeless earth loop measurementsStakeless earth loop measurements

Rx R1 R2 Rm-1 Rm

Rx

Rx R1 R2 Rm-1 Rm

Rx Rn= m earth resistances in parallel


Earth Testing Methods (4)Principle: Stakeless earth measurementsPrinciple: Stakeless earth measurements

Erder

black

red

• For all applications with at least two earth electrodes...e.g. high tension towers, meshed grounding systems,

• telecom-cables ......

Simply put both clamps around conductor, cable, pipe etc.

Clamps with a large variety of openings are available.

> 10cm

Earth electrode

Adapter – for GEO X only



Example: Stakeless earth measurementExample: Stakeless earth measurement

earthing / grounding issues

Documents