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Topic Title: Cooker supply
Topic Summary: Odd readings?
Created On: 03 December 2013 09:05 AM
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 03 December 2013 09:05 AM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

Chaps,

New cooker supply with socket on a TN-S via service block/RCD module and MCB.

R1/R2 measures 0,56 ohm.
Ze seems to be fluctuating between 1.57 and 2 ohms. DNO informed.
Measured Zs 31 ohms?

Any thoughts why the measured Zs is so high?

S.
 03 December 2013 09:22 AM
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daveparry1

Posts: 6239
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It can surely only be a bad connection somewhere, or possibly a faulty rcd putting a h/r on the live maybe?

Dave.
 03 December 2013 09:47 AM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

Surely a bad connection would present itself during the R1/R2 measurement?

I checked at both cooker termination and at the CCU socket. Same readings.

RCD trip tests are fine.

S.
 03 December 2013 09:58 AM
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perspicacious

Posts: 7241
Joined: 18 April 2006

"TN-S"

Original sweated connection to sheath or a BS 951 earth clamp?

Regards

BOD
 03 December 2013 10:06 AM
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AJJewsbury

Posts: 11552
Joined: 13 August 2003

Surely a bad connection would present itself during the R1/R2 measurement?

Not if it's upstream of the MCB.... (e.g. on the DNO side).
- Andy.
 03 December 2013 10:10 AM
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SherlockOhms

Posts: 325
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This job involved a service block so main fuse removal.

The service head was in a bad way so I requested the DNO attend.
They did and replaced the service head.

Surely they would have checked the Ze?
 03 December 2013 10:23 AM
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AJJewsbury

Posts: 11552
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Surely they would have checked the Ze?

Last time I witnessed any supplier work, their complete testing consisted of a 'socket-and-see'.....
- Andy.
 03 December 2013 10:33 AM
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aligarjon

Posts: 2867
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I'd also try taking a zs either side of the rcd to check that and on the terminals of the cooker outlet or isolator rather than plugging into the cooker socket if you took the reading that way. Also check your tester leads. Is the circuit breaker tightened up properly, take a reading from the outgoing terminal of that ?

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 05 December 2013 12:30 AM
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SKElectrical

Posts: 910
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I don't understand the point of the non trip function on a earth loop test meter. It always gives an inaccurate answer, and if Ze isn't low then the permitted Zs is often exceeded.
 05 December 2013 05:55 PM
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KFH

Posts: 214
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Using a MFT 1530 on a TNS supply with high Ze, I got a variety of high Zs readings on the no trip setting. Once DNO had fixed supply the no trip readings were fine. I assumed the fault on the earth was causing interference on the no trip readings.
 05 December 2013 05:56 PM
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daveparry1

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Not if you use a Fluke SK! The inaccurate readings given by lesser meters are due to using a low current test, the Fluke uses a different system (very short test duration, not long enough to trip rcd) so doesn't suffer from the problem,

Dave.
 05 December 2013 07:50 PM
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breaker

Posts: 280
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Have you tested any other circuits in this property to see what result you get?
 05 December 2013 10:01 PM
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JonSteward

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Are you measuring with the load connected?
I would disconnect the oven and test again.
 06 December 2013 08:07 AM
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SherlockOhms

Posts: 325
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No I never tested any other ccts in the property.

I did the test with the oven disconected.

Could some kind soul explain what causes the raised Ze value to confuse the test meter? How does it distinguish between a high Ze on a TN-S and a TT which would be normally high?

S.
 06 December 2013 12:57 PM
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perspicacious

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Original sweated connection to sheath or a BS 951 earth clamp?

Regards

BOD
 08 December 2013 12:06 AM
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stateit

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Originally posted by: perspicacious

Original sweated connection to sheath or a BS 951 earth clamp?

Regards

BOD


Thinking about it, I don't think I've ever seen a sweated connection down here in the (historically) SEEB area. Even on very old installations.

Always been either clamps (two part, screws either side) onto the lead, or, on my own 1948 house install, a buckle strap pulled tight.

-------------------------
S George
http://www.sg-electrical.com
 09 December 2013 11:24 AM
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AJJewsbury

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Could some kind soul explain what causes the raised Ze value to confuse the test meter? How does it distinguish between a high Ze on a TN-S and a TT which would be normally high?

I suspect a loose connection with tarnished/dirty/corroded metal - with a decent test current the tarnish breaks down and the joint seems to have a relatively low resistance, but with a mA test current the tarnish remains intact and blocks current like a higher resistance. A bit like a crude semiconductor if you like.
- Andy.
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