Earth Loop Testing (1/3)

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Jun 21, 2008
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Right, not much seems to be happening tonight so I'm going to ask you some questions. Now I'm interested in this testing you talk of, and from what i've picked up seems to involve Earth Loop Impedance, Insulation test, and testing of the MCB.

In terms of Earth Loop Testing - is it really just ensuring that each part of the circuit is continuous with the earth? (a sort of continuity test so to speak)?

Surly something is ether connected to earth or not and so how can we put numbers to it? Ze

When testing an installation, say with a CU change, WHERE do you measure it? Each fitting? Each circuit? Once?

Bonding is earth to incoming gas and water (unless in plastic) - is this tested too?

What about supplimentary bonding? Do we make sure the bath taps are earthed?

I can imagine it's different depending on the earth arrangement (rod = TT, from electricity company = PME)

(I'll ask about the other 2 parts another night)




Senior Member
May 28, 2008
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Zs = Doing earth loop tests is to verify that the resistance is low enough to allow in the event of an earth fault, enough fault current to flow to cause the protective device to operate within the correct time. Tested at the furtherest point from the CU on each circuit.

Equipotential bonds should be tested to ensure that they are continuous and connected to the MET and the extraneous part. usually using the long lead method.

Ze = Is similar to the Zs but is taken from the main isolator, switched of and the main earth conductor removed from the MET the Ze is the resistance of the incoming supply and the main earth connection to the point of earth PME on a TNCS system around .35 ohms / the outer sheath on the supply cable on a TNS system around .8 ohms / and Earth on a TT system has to be less than 200 ohms, hope this helps CJS:)

Mar 7, 2008
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In the office again.
Just to expand on CJS - r.e. earth loop (Zs/Ze/Ra).

The measurement taken (Ze) is the resistance of the (phase) supply tail, through the meter & cutout, down the supply cable to the transformer (substation), through the (earthed) neutral point and back to house via PEN conductor (TN-C-S) or cable sheath (TN-S). Zs same but includes furthest point of final circuit wiring (for EACH circuit) within the property.

Ra - similar, but return path is through the "general mass of earth", and your rod, tape, structural metalwork etc. et al, et al.


p.s. CJS - like your new word m8 - furtherest. Can we also have "most furtherest, or does that imply that there may be a " least furtherest"?

Admin: can we have a subsection, relating to sparkys` unique words, the definitions and origins? I know what dog c0xs are, but not why they`re called that. :eek:

Mar 28, 2008
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Too expand a bit further... and pick up point from your first post..

In terms of Earth Loop Testing - is it really just ensuring that each part of the circuit is continuous with the earth? (a sort of continuity test so to speak)?Surly something is ether connected to earth or not and so how can we put numbers to it? Ze
As CJS & KME pointed out the ZS is the earth fault loop impedance which you take at the furthest point of every circuit..

This values MUST be within the correct limits specified in section 411 of the wiring regs, (various tables for Different fuse types & circuit types) pages 48 / 49.

as ZS relies on a reading up Live/Phase/Line conductor & down Earth/CPC,

as you quite rightly observed it necessitates a CONTINUITY being present on the earth wire.

The earth continuity can be verified 'prior' to doing ZS tests by doing continuity reading of R1(live) & R2(earth)

On a new circuit this is one of the "Dead" tests that are done before circuit is energized

When doing R1 & R2 the conductors are looped at the fuse box and reading taken at furthest point on the circuit.

A table in the On-Site-Guide gives values for resistance per metre for all the standard sizes of copper cable..

So if you have your R1+R2 values & you know what size cable you have used...

If the values is a lot higher than the expected value..

it means you probably have a bad joint or some damaged cable somewhere!

Also as the others said Ze is the "external" bit of the ZS reading..

i.e. the earth loop impedance at the fuse box.

Now this is where a bit of a basic easy peasy formula come in!!


So if you have a reading for Zs & Ze you can work out R1+R2...

as well as checking it is within the max values in the regs..

you can check the "apparent cable length" appears ok..

For example:-

Cooker circuit,

6.0mm cables, (2.5mm earth)

32A Type B MCB.

Max Zs permitted = 1.44ohm (from table in regs)

Say Ze is 0.20ohms (assumed figure say TNCS / PME installation)

If we got a reading of say 0.95ohm for Zs

Zs-Ze = 0.95 - 0.20 = 0.75ohm so R1+R2 = 0.75ohm.

From O-S-Guide 6.0mm/2.5mm Cpc Pg 158, table 9A

R1+R2 = 10.49 miliohm/meter

(0.75/10.49)x1000 = 71.5meters Approx

now if you knew the cooker was less than 30m from the CU..

you should start thinking

eh-up we may have a problem with the cable here???

has the circuit been altered with a dodgy joint or wrong size cable???

for 30m of 6.0mm, R1+R2 should be approx 0.31ish.


so ZS should have been no greater than.. Ze+R1+R2

=0.20+0.314 Max Zs 0.514!

So you can have a Zs that appears correct for MCB tables but is too high for the actual cable length.

Hopefully that may clear it up a bit more how they all inter-relate with one another.. :D

sort of same thing but different but all very important and useful! ;) :)

or.. :|

maybe I just fried your brains a bit... :eek:

Hope me calculation all appear OK...

brain getting a bit fuzzy.. but have done it slowly..

so hopefully I am OK..

If not... :(

I am sure KME will pop over and tip a bucket of Theory's pig-swill over my head!! :eek: :( :^O :^O



Forum Founder
Feb 6, 2008
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Your examples helps to make this forum what it is, Specs.

Thank You. :x



Senior Member
Feb 16, 2008
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When doing your testing dont solely use the Zs-Ze formula to work out your R1+R2 because this doesnt prove continuity. Just use it to check figures. :)