ANOTHER WEIRD ONE!!!!High ZS readings

mgnelec09 · Sep 15, 2011

Hi There,

We have just been testing a job that is coming to a close.

The job is a detached bungalow out in the sticks.

The supply comes down overhead lines then down the column, then underground into the property.

Im pretty sure its a PME system.

My end to end readings on the ring mains are pretty normal and R1+R2 also.

My Zs readings on all 4 ring mains are pretty high.

all reading around the 1.00ohms mark although some of them are up to about 1.5 Ohms.

My ZE reading at the supply is 0.47 Ohms

I rang Niceic Technical advise as I wasnt happy with the readings knowing that they need to be under 1.15 for a type B 32A breaker.

The guy told me I need not worry as long as it is protected by an Rcd, and as long as the RCD tripping times are ok.

He then went on to say that it can be anything upto 6000 Ohms!!!!!! Can some one explain wot hes going on about!

Cheers

telectrix · Sep 15, 2011

if it's PME then your Ze is way over the max. of 0.35 for a start... with RCD protected circuits, your max. Zs is 1667 ohms as then the RCD will act as fault protection should the Zs be too high to trip the OCPD within the 0.4 secs.

Paul.M · Sep 15, 2011

I'm no expert in testing so please correct me if I'm wrong but:- R1+R2 = 1 Ohm, Ze = 0.5(rounded) and your getting a Zs of 1.5. What's the problem?

With an rcd you can have up to 1667 Ohms (dont know about 6000 Ohms or is it 6KA that he told you?)

Ok lads I might be wrong off the top of my head, please say so if I am so I can learn where I'm going wrong.

Paul.M · Sep 15, 2011

paul.m said:
I'm no expert in testing so please correct me if I'm wrong but:- R1+R2 = 1 Ohm, Ze = 0.5(rounded) and your getting a Zs of 1.5. What's the problem?

With an rcd you can have up to 1667 Ohms

I've just re-read the op, pme? Overhead supply in the sticks? Readings are high for a TN system but very low for a TT, need to know.

mgnelec09 · Sep 15, 2011

Thanks telectrix, should I record the exact readings on the test sheet? with it being overhead lines would it be a TT? I know a TNC-s goes upto about 0.8 ohms. the cable got jointed as the house was being re-built and moved about 3 metres by an old elecy board specialist. I wasnt there when he did it. could this have any influence on the reading? The resin joint looked ok.

Is there a reg that says about high values and Rcd protection? And is the 1.15 for a type B 32amp only for non rcd protected circuits?

telectrix · Sep 15, 2011

first thing is to measure PSCC and PEFC. if they're the same it's PME (TNC-S). it's TN-S that can be up to 0.8.and as paul says. need to know what it is. guessing is for morons on family fortunes.

Rockingit · Sep 15, 2011

By the sounds of it, it's more than likely to be TT in which case your Zs can run up to 1667 as others have said. The difference is that you MUST have RCD protection on all circuits to compensate.

See Reg 411.5 and table 41.5

A property I went to a little while ago was fed off a pole with a PME stamp on it, but it was still TT at the house end - guess someone was being a cheapskate with the cable when it all got built!