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.

 

[Octopus]

Poor connection(s)

 

[Mark_Burgess]

As above, good look in the isolator switch (if one fitted) and maybe turn it on and off a dozen times.

 

[telectrix]

try taking a reading either side of the MCB . it could be the MCB affecting the reading.

 

[needasparks]

Well spotted, so many people just put readings down without any thought in what reading they are likely to expect to get.
sounds to me more likely a poor conection when the test was made unless your testing Zs at a different place to R1&R2.

 

[sjm]

I'm pretty damn sure about all the connections. New isolator fitted, Crabtree 50 amp ceiling switch.
Should have said that I'm getting similarly high readings on the cooker circuit and radial socket circuit. I'm fairly sure that the meter is not working properly on the no trip setting, but just wanted to run it past you guys.

Thanks

 

[Amp David]

Test ZE with bonding connected, you've already done that i think when you state Zs at CU. Then test on the incomming side of RCD with Hi and no trip, then on the supply side of the RCD on no trip,

Bet its the RCD or no trip causing it. If you have 2 RCDs I bet you'll get a substantial difference between the 2

 

[Octopus]

How well charged was the battery in your tester?

How old are your tester leads?

Did you null out the leads before the tests?

 

[telectrix]

if he's using a megger, no need to null leads on EFLI readings. bet the readings differ across RCD and/or MCB.

 

[needasparks]

It may be worth doing a long flylead test on R1 & R2 separately to a random circuit just to eliminate any mistakes in the test.
Are there any major voltage fluctuations that you have spotted ? near factory area etc. as with the quite low Ze it sounds like you are near to the transformer and you don't mention what earthing supply this is.

 

[tony mc]

IMO I would be inclined to say that this is the resistance of the RCD putting up the readings when you are useing the no trip on the meter.

Putting only a 15mA test current from the line to cpc as opposed to 25A on the Hi loop test does affect the readings.

Try linking out the line and cpc at the shower and doing a Hi 2 wire test at the CU without it going through the MBC or RCD and see what reading you get!

As mentioned its good to question the values that you get !

 

[vernam616]

Do a zs at the DB on no trip and see how it differs

 

[malcolmsanford]

I often think we sometimes over think things to be honest.

Though with the tests for Ze and R1+R2 the OP expected results of 0.25 but in fact got 0.91.

Well to be honest I think I could live with a 0.91 reading for what seems likely to be a 30mA RCD.

Remember we have resistances in the protection devices, and in 40 yrs I don't think I have ever measured anything that on paper it should have been. I do applaud conscientiousness, but only to a point.

0.91 ohm .......next please

 

[needasparks]

I often think we sometimes over think things to be honest.

Though with the tests for Ze and R1+R2 the OP expected results of 0.25 but in fact got 0.91.

Well to be honest I think I could live with a 0.91 reading for what seems likely to be a 30mA RCD.

Remember we have resistances in the protection devices, and in 40 yrs I don't think I have ever measured anything that on paper it should have been. I do applaud conscientiousness, but only to a point.

0.91 ohm .......next please

I think the point being made is how reliable is that reading of 0.91 if the earthing has tripled, is it going to become non-existant soon. I would want to know why it has leaped so much. I understand what you are saying though but i would be worried i had missed something obvious and possibly major later.

 

[malcolmsanford]

What we got to remember is the tolerances we are working with here, on MFT that really are not designed to be that accurate when down to 0.1 of ohms as we had with the Ze and the R1 + R2.

We don't know about lengths of cables, the size of cable, protection devices influences, or even what difference the voltage RMS was on when taking the Ze and the Zs, there are so many factors but a Zs of 0.91 compared to a "calculated" 0.25, at these tolerances, well put it this way I wouldn't be losing much sleep.

If I got a measured Zs of 2 point something, then yes I would be a little concerned.

 

[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.