ZS in a PME earthed household

[Darkone]

Please be gentle with me it's been a long day.

In my previous life I was an electrician, suppose I still am but not practicing installation work.
So I am trying to get up to speed with some modern practices, mainly Earth Loop Impedence.

In my daily work I constantly use a megger for safety testing but also have an Earth loop tester, which i thought I was quite converse with its use, but a situation today made me want to clarify my readings.

So the situation is that on testing a 13a socket outlet on a ring main in a utility room I got a reading of 1.57ohms, the next socket approx 2 metres away was 0.47.
I checked other sockets in the kitchen area and got readings from 1.48 to .53ohms

I suggested to the house owner that these readings have, 1, too high a reading and 2 should not have such a varience on the same rings.
The householder found the wiring certs isued by the contracting electrician so I checked these.

These are some of the details recorded:
PME earth as source (ZE = 0.32ohms)
Max allowed ZS for kitchen ring 2.6ohms? .... measured 1.48
Kitchen ring on 32a 60898 mcb, not sure of type(BCor d)

Now I always believed that Bs En6098 and Bs 3871 MCB should have a max ELI readings in a PME installation of less than these readings ie type 1=1.44ohms 2=.82ohms B=1.15ohms C=.57ohms etc

So am I loosing the plot or has the contracting electrician made errors both in the installation and certificating, and a Niceic sparky at that

 

[GaryM]

Firstly, did you measure Zs at the terminals behind the socket or through the pins with an adaptor?

Maximum, uncorrected figures:

B32 - (230/(5*32) = 1.43ohms
C32 - (230/(10*32) = 0.71ohms
D32 - (230/(20*32) = 0.35ohms

 

[telectrix]

your max. Zs figures are correct. his are wrong. the reason you are getting variations in the readings from socket to socket are. 1. poor connections. 2. poor sockets, internally.3. some sockets may be spurs.

 

[rich.250]

Zs will vary around the ring due to there not being an interconnection as you would with your R1+R2
I would always test the socket from an adaptor to prove Polarity!
Try plugging it in and out a few times to clean the Contacts up

 

[jaresquire]

Zs will vary around the ring due to there not being an interconnection as you would with your R1+R2
I would always test the socket from an adaptor to prove Polarity!
Try plugging it in and out a few times to clean the Contacts up

Surprising how rough (ie high resistance) some sockets can be, no make seems to be immune.

 

[Darkone]

I measured ZS by pluging meter via its 13a plug top straight into the sockets I tested. Yes I did pull the plug in and out a few times as I have been caught out before with poor sockets. This is new build about a year ago so I cannot see how corrosion will be an issue. My money is on a loose connection. Electrical contractor has been notified so I await a phone call from him.


I just want to get my facts right first :2guns:

 

[GaryM]

If you're seeing abnormal results through the front of the socket then you need to take a measurement directly from the terminals at the rear to rule out other problems or highlight a socket with higher than normal resistance contacts.

 

[nickblake]

Please be gentle with me it's been a long day.

In my previous life I was an electrician, suppose I still am but not practicing installation work.
So I am trying to get up to speed with some modern practices, mainly Earth Loop Impedence.

In my daily work I constantly use a megger for safety testing but also have an Earth loop tester, which i thought I was quite converse with its use, but a situation today made me want to clarify my readings.

So the situation is that on testing a 13a socket outlet on a ring main in a utility room I got a reading of 1.57ohms, the next socket approx 2 metres away was 0.47.
I checked other sockets in the kitchen area and got readings from 1.48 to .53ohms

I suggested to the house owner that these readings have, 1, too high a reading and 2 should not have such a varience on the same rings.
The householder found the wiring certs isued by the contracting electrician so I checked these.

These are some of the details recorded:
PME earth as source (ZE = 0.32ohms)
Max allowed ZS for kitchen ring 2.6ohms? .... measured 1.48
Kitchen ring on 32a 60898 mcb, not sure of type(BCor d)

Now I always believed that Bs En6098 and Bs 3871 MCB should have a max ELI readings in a PME installation of less than these readings ie type 1=1.44ohms 2=.82ohms B=1.15ohms C=.57ohms etc

So am I loosing the plot or has the contracting electrician made errors both in the installation and certificating, and a Niceic sparky at that

im with Telextic on this one sounds like poor connections if the MCB is a B type 32a then the max recorded Zs is 80% of the max allowed in BS7671 which should be so max Zs in Bs7671 is 1.44 so the reading should be around 1.15

 

[steveberry11]

Have you carried out all the ring checks. End to end and figure of 8.
The problem may be that you do not have a complete ring and you are testing one long radial and one short one
if you see two cables on top of the MCB do not assume that it is a ring CHECK.

 

[Darkone]

Have you carried out all the ring checks. End to end and figure of 8.
The problem may be that you do not have a complete ring and you are testing one long radial and one short one
if you see two cables on top of the MCB do not assume that it is a ring CHECK.

No these are ring main circuits, ok there may be an odd spur involved but I cannot see double sockets on a spur. I have no need to do end to end figuire 8 tests, that should have been done by the contracting electrician. I have only needed to do a ELI test to confirm a presence of a good earth and measurement, as per my first post.
I use my ELI instruments several times a day in different property's, in general most are good and within the prescribed tables. I was just asking for confirmation of what I have proven to be right, poor certification by contractor.

 

[Lou7]

How do you know its a ring without testing it ?

 

[Darkone]

How do you know its a ring without testing it ?

Perfectly true one cant, that fact that the consumer unit and certification only has “ring mains” marked for the socket circuits suggests to me that they are, although do I trust what has been written?
The first socket I tested was only about 7-8 meters from the consumer unit so even if it is a radial circuit then I would still expect the ELI to be similar or close to ZE, should it not?

 

[steveberry11]

No these are ring main circuits, ok there may be an odd spur involved but I cannot see double sockets on a spur. I have no need to do end to end figuire 8 tests, that should have been done by the contracting electrician. I have only needed to do a ELI test to confirm a presence of a good earth and measurement, as per my first post.
I use my ELI instruments several times a day in different property's, in general most are good and within the prescribed tables. I was just asking for confirmation of what I have proven to be right, poor certification by contractor.

1st. You do not know that they are ring mains for sure unless you test.
2nd why do you assume someone else tested correctly
3rd you do not need to do the end to end or figure of 8 test on a condition report if there is no evidence of additional work having taken place on the ring
4th. If this is an addition to the ring main how do you know you have installed it correctly unless you do a full ring test
5th. If this is an addition to a ring with a high or unsuspected loop you must carry out a full test to locate the cause of the odd readings for the test cert
6th When the EIC was completed all may have been ok you may have found a developing fault that should de reported or investigated.

 

[Darkone]

Thanks Steve. We seem to be off an a tangent to the original posting, even I have had to read it again.

To reitorate I was only testing for a sound earth and expected a ELI reading more inline with what I would expect knowing the ZE is low, being recorded as 0.32ohms on contractors certificate and my own testing at the consumer unit 0.45ohms via a socket next to it.
As said previously, the property is only about a year old so should there be any devloping faults? And yes my concerns have been brought to the householders attention, that’s why they need to get back to the Niceic contractor.

 

[nickblake]

you would be suprised the differance testing across the supply and the socket , there could be a slight resistance across the socket 0.13 is not much this reading can also change due to the ambiant temperature when it was originally tested it could have been below freezing and your testing when its warm these can effect the readings , just a thought