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Topic Title: Earth Loop Impedance
Topic Summary: Electrical Testing
Created On: 20 April 2009 07:41 PM
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 20 April 2009 07:41 PM
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dproctor1882

Posts: 4
Joined: 20 April 2009

I am currently carrying out electrical testing on the machines in my workplace. When I was doing the earth loop impedance on a 3 phase 32A k type breaker, I was getting readings of around 1 ohmn on each of the phases. From table 2D in the 17th edition the maximum impedance for this breaker is 0.29ohmns and with 3/4 rule of thumb applied 0.22ohmns. I check this on the rest of the machine and none of them were anywhere near 0.22ohms. The circuit breaker is connected to a 30mA RCD, so I would like to know if you have an RCD built in with the circuit breaker do you not have to achieve the earth loop impedance for the disconnection time and instead rely on the RCD for fault protection?
 20 April 2009 07:45 PM
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Ricicle

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Joined: 23 October 2006

What do you mean by 'k' type breaker ?
Are these readings being taken at the supply terminals to the machines ?

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 20 April 2009 08:05 PM
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dproctor1882

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Joined: 20 April 2009

K type breakers are the same as the D type on table 2D, (D for power supplies and K for motors).
Readings taken at both circuit breaker, and also measured at supply terminals and both were around 1ohmn.
 20 April 2009 08:12 PM
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AMN

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Are the high Zs readings due to a high R1+R2 and or high Ze/Zdb ?

AMN
 20 April 2009 08:29 PM
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dproctor1882

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Have not got a chance to investigate ZE at distrbution panel supplying the four machines or the resistance of the cables to the isolators coming to the machines because they are kept running most of the time. It's whether or not having an RCD connected to the circuit breaker enables me to forget about the earth loop impedance maximum impedance.
 20 April 2009 08:44 PM
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Ricicle

Posts: 841
Joined: 23 October 2006

But if it is caused by high Ze/Zdb then what about any circuits without RCD ?

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Empty barrels make the most noise.
 20 April 2009 09:10 PM
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ebee

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Some testers use a lower current on the RCD setting when doing the Zs test than they do on the Non RCD setting and give quite different results.

Try measuring ZS on a non RCD cct with the meter set at both the RCD and non RCD settings and see if you get significantly different readings.
Some meters also have a lower resolution on this setting too.

In days of yore it used to be suggested to temporarily bypass the RCd for the Zs test (I`m not suggesting you do this).

If you`re happy the RCD will trip every time then no problem but if you`re worried about the possible 7% failure rate of RCDs then well mmmm. Mind you on a TT install you`d have no other choice.

Whan are the Ph to N loops or Ph to Ph reading?

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Edited: 20 April 2009 at 09:13 PM by ebee
 20 April 2009 10:23 PM
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mark2spark

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I'm pretty sure you'll need to test by taking the rcd out of the frame.
Or get a tester with anti trip technology like this

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 20 April 2009 10:32 PM
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perspicacious

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"Ph to Ph reading?"

My "loop" tester doesn't like 400 V across it

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BOD
 20 April 2009 10:51 PM
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kj scott

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When you say that you are testing machines; are there motor overloads in circuit? As this will give higher loop readings.

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 21 April 2009 03:08 PM
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deleted_1_dproctor

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Kj Scott, tested at the supply terminals with the machine isolator off, still 1ohmn. Is it to do will multiple earths?
 21 April 2009 03:14 PM
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deleted_1_dproctor

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Had another look through the 17th edition book and found table 41.5 which gives a maximum impedance to ensure RCD operation of 1533ohmns (TT system)? We have a TN-C-S system so the earthing will be far better than TT valve.

Should the circuit breaker still have to meet maximum impdance values even if using an RCD in case of RCD failure?
 21 April 2009 03:21 PM
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deleted_1_dproctor

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Ricicle
But if it is caused by high Ze/Zdb then what about any circuits without RCD ?


All other earth fault loop impedance tests were OK because they were 6A & 10A type 3 breakers which had maximum impedances of 3.09ohmns & 1.85ohmns, this 32A type K breaker has a max of 0.29ohmns which I cannot acheive= >1ohmn.
 21 April 2009 05:12 PM
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AJJewsbury

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What do you mean by 'k' type breaker ?


Have a look at page 10/2 of the ABB Catalogue

(instant trip looks to be between 10x and 14x In, so upper limit a bit lower than D type (between 10x and 20x))

Had another look through the 17th edition book and found table 41.5 which gives a maximum impedance to ensure RCD operation of 1533ohmns (TT system)?


Close - 1667 ohms for 30mA - if you're on a normal 230/400V system (Uo <= 230V).

Should the circuit breaker still have to meet maximum impdance values even if using an RCD in case of RCD failure?

Not necessarily - regs allow RCD to provide sole earth fault protection (411.4.9).

But I think many here would be concerned with such high Zs values on a conventional TN system. If the loop impedance if that high, then either there's a bad connection somewhere or if it's due to cable length etc, you're probably in trouble for voltage drop.

- Andy.
 21 April 2009 09:00 PM
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dproctor1882

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Joined: 20 April 2009

Cheers lads for all the info, going to measure Zdb and r1+r2 of the supply cables tomorrow to investigate high impedance values.
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