Zs reading high!

[sjm]

Hi guys, what do you think of this?

Ze reading of 0.13. Zs reading at the CU of 0.12 with bonding attached etc.
Shower supply R1 + R2 reading of 0.16. Zs reading at the Shower 0.91.
Using my new Megger LRCD220 meter on 2 lead Hi setting for Ze and Zs at the board.
Then using it on No trip setting for Zs reading at Shower.
Can anyone think of a reason for the incorrect reading, or is the meter US as I think.

Thanks in advance.
Steve.

 

[telectrix]

that reading of 0.91, thouhg, is borderline if on a 40A MCB.

 

[tony mc]

Malcolm did say likely to be 30mA RCD so I guess the 1667 now comes in Tel!

And with the no trip use I would say it has a Rcd on the cct!

 

[telectrix]

agreed, but i always strive to get a Zs value low enough to use the OCPD for fault protection.

 

[Rockingit]

agreed, but i always strive to get a Zs value low enough to use the OCPD for fault protection.

Me too. And my guess is that the OP is really just wanting to know if his new MFT is working properly or not.

 

[sjm]

Thanks for the replies guys.
Absolutely right Rockingit. I will not be happy having spent £500 on a new Megger if I can't trust it's readings, or get readings that are above the max Zs allowed by BS7671 when it shouldn't be. Yes there are tolerances for test results but +/- 5% not + 250%.
To clear up a couple of matters the supply is TNCS, the length of cable is about 20 metres (8 metres of 6mm T+E inside cavity wall upto loft, then into 60 amp juntion box joined to 10mm through isolator to shower.) Yes 0.91 is just OK for the 40 amp MCB but it's very close.
As I said earlier I also got high readings on the cooker circuit 101 when it should have been 0.32 and radial sockets 106 should have been 0.53.
I'm starting to think I may need to send the meter back to have it checked.

 

[telectrix]

first compare it with another meter if you have a mate with one. i would still be inclined to suspect MCB or RCD is adding that 0.7 ish value.

 

[needasparks]

first compare it with another meter if you have a mate with one. i would still be inclined to suspect MCB or RCD is adding that 0.7 ish value.

thats a good point, the meter may go out of cal above a certain reading.

 

[Heisenberg]

And one of these can help, if you start to get readings that you think are dodgy..

TestCal CalCard — Calibration Resistance Checkbox

 

[WDMDL]

MCBs have to have some impedance in them... for the heating element that bends the bimetal for the low%, long duration overload and of course the electromagnetic trip coil.
In practice, the small amount these add to the measured Zs is negligible, as the resistances are inversely proportional to current rating.

I have found a good number of RCDs that add significantly to Zs values though. BG is one of the brands that spring to mind.
Yet most brands don't.
I have a bit of a theory though.
Think about how an RCD operates, it's a transformer with two primaries 180 degrees out of phase, so if what goes out down Line comes back down Neutral, the magnetic flux and therefore the inductance cancels out (you've just got winding resistance) and importantly, there's nothing coming out of the secondary.
But under the Zs test, you're causing a deliberate imbalance. Therefore there will be a measurable inductance, it is this which adds to the system impedance Zs.

I had one last week - a SFCU for a boiler. 20A Radial circuit, 2.5/1.5T&E 1m long.
Nice beefy TN-S supply, Ze 0.07 Ohm, PFC 3.9kA
(R1+R2) 0.01 Ohm.

Was the Zs 0.08 Ohm? No. It was 0.57 Ohm.
Testing on the supply side of the RCD got a Zs of 0.08 - but on the load side 0.53 Ohm - both on No Trip setting.

Simon.

 

[Deleted member 9648]

Thanks for the replies guys.
Absolutely right Rockingit. I will not be happy having spent £500 on a new Megger if I can't trust it's readings, or get readings that are above the max Zs allowed by BS7671 when it shouldn't be. Yes there are tolerances for test results but +/- 5% not + 250%.
To clear up a couple of matters the supply is TNCS, the length of cable is about 20 metres (8 metres of 6mm T+E inside cavity wall upto loft, then into 60 amp juntion box joined to 10mm through isolator to shower.) Yes 0.91 is just OK for the 40 amp MCB but it's very close.
As I said earlier I also got high readings on the cooker circuit 101 when it should have been 0.32 and radial sockets 106 should have been 0.53.
I'm starting to think I may need to send the meter back to have it checked.

The no trip test on the new 2 lead meggers IS totally unreliable....do 10 tests on the same point and you will get 10 different readings. I sent my megger back after a week,and it came back saying nothing was wrong....they even suggested I didnt know how to test. I understand the point about resistance through RCD's affecting it,but given that it is designed to be used on circuits with an RCD this is hardly a valid excuse.Now I just factor in it's unreliability,use it with a combination of the expected calculated result to 'average' it out....if possible do several Zs tests and take an average from them. But as malcolm states dont lose any sleep over it.

 

[sjm]

Yes the board and RCD, MCB's are cheap Control Gear from Denmans, so quite possibly high impedance through the MCB's.
Interesting to hear from wirepuller that you were also concerned about your new Megger and sent it back for testing. At least I'm in good company if do the same. Mine is a combined loop and RCD tester with 2 wire Hi current loop test, but does require 3 wires for no trip test. It's pretty C**P if the supposed best meter manufacturers can't make a no trip meter that gives correct and reliable results.

 

[sambotc]

My Metrel does this, always way higher on Notrip function, presumed it was the resistance through the RCD but have to be honest, think its pretty pants that they cannot somehow offset the result (not sure how myself mind!) I was going to ask a similar question the other day about it actually but forgot, seems all the MFT's suffer from this then?

 

[i=p/u]

its the silly 15mA test..

back to college days when doing R1+R2 testing, you start off at say 2000 ohms get high reading, then drop scale too 200 ohms and get better reading, then drop scale too 20 ohms and get better reading again..

pretty much the same, your imitating a fault but not the same fault you need to dissconnect the mcb..

 

[IL1970]

your using the no trip setting for the zs at the shower? what are the readings for the zs at the board if you use it on no trip. I know with my fluke that readings on no trip are considerably higher than on normal setting

 

[sambotc]

Ok, so on the basis that all MFT's appear to give higher results on notrip setting, which result would you record? (R1+R2)+Ze or notrip Zs ? Are there any other options as otherwise its going to look like you've got a resistive terminal somewhere on the test certificate, or would you note that test was carried out using notrip facility and given the fact that an RCD is present the disconnection times are satisfactory?