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Topic Title: Martindale ET400
Topic Summary: any users?
Created On: 17 August 2015 04:21 PM
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 01 September 2015 04:49 PM
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leckie

Posts: 4320
Joined: 21 November 2008

And then what happens?
 01 September 2015 08:10 PM
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Zs

Posts: 3814
Joined: 20 July 2006

Ah, when you get everso close to the transformer and the EFLI is really only about 0.0001 or something (I made that number up), some testers aren't as good so they give you 0 ohms and you end up writing it on your certificates as 0.1 ohms because if you don't do that then all the maths goes wrong.

As you know, I'm not a huge fan of Megger but the more this goes on the more I realise that in fact their kit is quite good. You know that annoying clever bloke in your class - the one who wore his Rugby Ugly inside-out and had a bit of catenary wire holding up his jeans - who was a bit of a pratt but always did really well in exams because he actually knew his stuff? Hmm, that might be Megger.

You still with me?

You'd think that joining the N and the E together would do the trick and it probably does for most jobs. I used to do it with the Fluke. But speaking as someone who these days prefers to test L-E because that really puts pressure on the 'E' at least all the way back to the building's MET, then I have to say that 2 lead testing has been a revelation.

Using the N for a test? Well IMHO you might as well not bother. You know the circuit is working after all so no question, you have an 'N'. You know the supply is working so there's another bit of evidence for the N. Come face to face with an 'E' problem at the supply or half way along and you'd not even know about it with that test. I'd call that a drive-by, but in a posh car.

I don't approve of using the N as if it were an E - unless as part of a fault find. But that's personal so I'll not go on. I am sure someone will enlighten me if required. I retire appropriately to cross-legged behind the sofa but with a guitar and an iPod just in case nobody objects to my views. I do not approve of using the N or any spurious links between E and N to prove Earth Fault Loop Impedance. There. I said it.

I am still in correspondence with Megger about the disproportionate cost of repair of the 1730. It might take a while but I'll keep you informed.

Zs

Edited: 01 September 2015 at 08:25 PM by Zs
 01 September 2015 09:05 PM
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OMS

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But speaking as someone who these days prefers to test L-E because that really puts pressure on the 'E' at least all the way back to the building's MET, then I have to say that 2 lead testing has been a revelation.

Using the N for a test? Well IMHO you might as well not bother. You know the circuit is working after all so no question, you have an 'N'.


Although most circuits will work quite happily with no "earth" and a neutral earth fault present - for lost neutrals on Overhead networks we just used to dab the neutral to earth at the TT installation rod - sort of quasi PME - until we could find the downed neutral conductor

Neutral earth loop testing used to be a defined test as it allows you to really put some proper test current down that earth conductor - in the region of 1.5 times the circuit rating (but usually limited to 25A) for a significant time period

It has the advantage that there is no connection to the "live" mains and if that earth is missing, no one gets a belt if touching installation metalwork

It has the disadvantage that it is only a "proxy" test that assumes the neutral is either smaller or the same size as the line conductor

No trip testing in the milliamp range for a matter of milliseconds barely stresses the earth conductor at all - you need injection testing (or GB's golden spanner) for that - or a neutral earth loop test as a good approximation

I guess there'll be a few on here that remember the old Seaward N-E Loop testers

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 01 September 2015 09:34 PM
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Zs

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Joined: 20 July 2006

Well there you have, in one fell swoop OMS, hit on why I have reservations about testing down the neutral.

My first job when I decided to become registered. TT. We needed the supply moving and I'd heard about getting a TT converted to a PME (or TN-C-S as I thought at the time). I'd also heard that if you could see a black line coming down the telegraph poles in the street outside you were ripe and ready for a conversion. 2005. Crikey I must be due for a career change? Either that or this is it for good.

Jolly fat chap from the supply Co comes round. He picks up a four inch screw from the floor and sticks it into the neutral terminal of the supply head so that he can hang his N and E crocs from it to run his test. I was aghast.

His result was mighty different from mine and miles lower.

I mumble-swerved my way through his visit, got the conversion and came home to spend about three nights on Google trying to get my head around what he had done.

Yes. Fine what he did for establishing the conversion capability but no, not fine for when you call me in to run a professional Inspection and Test on your building which is a TN, or indeed a TT- in which case, albeit still humbly IMO, testing down the neutral from a light fitting or a socket would be a daft thing to do if you are supposed to be testing or proving the CPC.

I don't remember those Seaward loop testers OMS. Does that make me younger than you?

Zs
 01 September 2015 10:06 PM
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OMS

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but no, not fine for when you call me in to run a professional Inspection and Test on your building which is a TN, or indeed a TT- in which case, albeit still humbly IMO, testing down the neutral from a light fitting or a socket would be a daft thing to do if you are supposed to be testing or proving the CPC.



OK - to be clear, it doesn't matter what other conductor we put current down - we are still putting current down the CPC in a loop test - it's a loop

If you do a live to neutral loop test - that can be high current and endure for some time - having proved the neutral, we can then put high current around the neutral earth loop and do that happily for some time without imposing a line voltage on the earthing system - so the objective is met ?

I'm not suggesting that we go back to N-E high current loop testing (PME installs can be a problem) by any means - but the idea that the CPC is only tested in a L-E loop test isn't entirely true - any loop that contains the CPC will do the trick

I don't remember those Seaward loop testers OMS. Does that make me younger than you?


LoL - Nope - you still have to put up your hand when I shout "Hands up all those over thirty"

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 01 September 2015 10:21 PM
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leckie

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Joined: 21 November 2008

Hmm, what I was asking was, does Paul conclude that the Fluke was superior to the Megger or vice versa? My megger often reads 0.00 ohms near a sub-station. So does the Fluke do the same?

Measuring rn is in my opinion a good idea on radial circuits as well as rings.

Measuring ELI on a TT? Well just think, if you get a reading for Ze (Ra), of say 25 ohms. Then say you measure ELI of say 35 ohms on a radial. You have say a 30mA RCD, so the result is way under the max permitted Zs. Is the circuit all OK. I doubt it, if you measured R1+R2 and it measured 10ohms then you might not be thinking that is a very good result. So I reckon on a TT then it's important to make sure that the R1 + R2 measurement makes sense. And also the Rn measurement. We don't want loose neutrals do we?
 01 September 2015 11:40 PM
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mapj1

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Some interesting points raised in this thread. No, not the one about being old enough to remember testing at 25A being the norm (as opposed to it now being just the domain of ol' misery guts in the corner. )
Though I do wonder how may earths that pass the 10mA " is it broken?" test may actually go open circuit under a real fault, because its hanging by a thread somewhere along its length. Equally how many impendance readings that are higher than they would be , as they are made via one side of the sense coil of an RCD, which at 30mA or less looks like an inductor but saturates out and looks like a straight wire above a few amps.
Given that N and E very rapidly become interchangeable concepts on the DNO side of the fuse, the inability to do a 2 wire test (i.e. harvest instrument power from the loop being tested ) is a suprising omission, as it must surely be possible, and in effect when measureing near a PME NE link that is what is happening anyway.
Is it just that instrument power (drawn L-N) is comparable to the test current L_E - surely not ? Do we think the instrument uses the N-E offset voltage during a sneaky L-N loop tests to deduce something about the impedance of the live conductor part of the L_E loop test ?
In a sense itwould be no worse than a wander lead R2 test, using the N as the wander lead, but if that is happening it would be really nice if they said so.

A megger note on various metering techniques and likely accuracy limitations

-------------------------
regards Mike


Edited: 02 September 2015 at 01:09 AM by mapj1
 02 September 2015 12:37 AM
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alancapon

Posts: 6880
Joined: 27 December 2005

Originally posted by: mapj1
Some interesting points raised in this thread. No, not the one about being old enough to remember testing at 25A being the norm . . .

I still have a Megger LT5 which tests at 25A for 2.5 cycles. It has just passed its annual calibration test too!

Regards,

Alan.
IET » Wiring and the regulations » Martindale ET400

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