distributers earth above 0.8 for tn-s

[dave1]

Just tested the Ze at the consumer unit to find it reads 2.47 for a TN-S system. As the lead sheath was so close I also tested with the earth probe directly on the earth sheath, which also gave a high value of 2.44. Replacing the earth clamp on the lead sheath made no difference either.

Do I contact the distributor and inform them their earth is not adequate or do I install an earth stake and make it into an TT system

Any ideas???

 

[telectrix]

i'd contact as you said. 2.47 is high. should be around 0.2 ish

 

[cagedanimal2010]

i would contact distributer, they should be able to help

 

[Richard]

Yea, get on the phone and they should be over to sort it pronto.

 

[nickblake]

Max Ze for a TNS installation is 0.8 so with a reading that high i would suggest a supply fault ahd you should contact the distributors ,
i had one yesterday,doing a job for a local panto group ,oh yes i was , flourecents on in the hall , lights worked ok , put the kettle on all the lights flickered just like ia disco , ah neutral fault so switched every thing off and did a test got a reading of 13.53 ohms ,scottish PWR with JCB's all day today and still not found the fault

 

[Guest123]

Just tested the Ze at the consumer unit to find it reads 2.47 for a TN-S system. As the lead sheath was so close I also tested with the earth probe directly on the earth sheath, which also gave a high value of 2.44. Replacing the earth clamp on the lead sheath made no difference either. You should not be putting as BS 951 clamp on the lead sheath of a supply cable!

Do I contact the distributor and inform them their earth is not adequate or do I install an earth stake and make it into an TT system

Any ideas???

As the DNO have supplied you with an earth connection, it is their responsibility to maintain it under the ESQCR. Give em a ring, although dont be surprised if the have a right moan at the earth clamp on their supply cable!!

 

[Deleted member 9648]

Just out of interest has anyone ever blown up a lead sheathed supply cable putting a clamp on it...or heard of an instance?.....Iv'e seen plenty of 951 clamps on the incomer and cringe every time.....

 

[nickblake]

Just out of interest has anyone ever blown up a lead sheathed supply cable putting a clamp on it...or heard of an instance?.....Iv'e seen plenty of 951 clamps on the incomer and cringe every time.....

nope but would be intresting to see how tight it would have to be to blow it up ....... u tube here i come .. no im joking !!

 

[Harry Manders]

Similar situation recently with Ze of 4.5 on a TNS system. Probably originally used water pipe to achieve good earth but not any more! Contacted distributor and they refused to fix for free so I needed to install earth rod and make installation into TT

 

[markthespark]

I have a query about high Ze on a TNS system if your Ze is 4.5 which is obviously too high. How come we have to TT the installation as if we TT's it our earth road resistance would probably be higher than 4.5 ohms?

Why cant we just install an S type RCD on a TNS system that is higher than 0.8?

Same goes for a PME system if our readings are higher than 0.35 why cant we just install an S-type RCD?

Cheers

Cheers

 

[WPD Lad]

I have a query about high Ze on a TNS system if your Ze is 4.5 which is obviously too high. How come we have to TT the installation as if we TT's it our earth road resistance would probably be higher than 4.5 ohms?Why cant we just install an S type RCD on a TNS system that is higher than 0.8?Same goes for a PME system if our readings are higher than 0.35 why cant we just install an S-type RCD?CheersCheers

If the DNO have given the customer a proper earth then they will be obliged to keep it, don't do anything just call them up. If its not a DNO earth then you'll either have to apply for one or TT it.The 0.35 limit is based on a 100A supply I believe this can be increased if the fuse size is reduced

 

[Knobhead]

A/ why resurrect a 3 year old post?

B/ get on the phone to get the DNO to sort their mess out.

C/ call a plasterer to hide the short comings.

In all seriousness you can’t hide a fault within the supply by fitting S type RCD’s. It really has to be sorted.

Yes I can see where your thinking comes from. But even for me it’s to close to the edge.

 

[markthespark]

Hi thanks for the replies, apologies for resurrecting an old post i was just having a browse through.

Yeah i agree with the point about hiding faults with RCD's

I was trying to work out the theory side of things, even though the Ze is above the allowed maximum, it is still going to be lower than a TT Ze so just wondered why it was the norm to TT installations that exceed the maximum Ze values on TNS and TNC-S systems.

Cheers

 

[malcolmsanford]

Hi thanks for the replies, apologies for resurrecting an old post i was just having a browse through.

Yeah i agree with the point about hiding faults with RCD's

I was trying to work out the theory side of things, even though the Ze is above the allowed maximum, it is still going to be lower than a TT Ze so just wondered why it was the norm to TT installations that exceed the maximum Ze values on TNS and TNC-S systems.

Cheers

The real bottom line to this is stability. Your quite correct the chances of you finding a TT system of less than 5 ohms is slim to virtually unheard of, but the protection system for a TT is designed slightly differently, well it was in the past.

Before the 17th there was only very certain exceptions for additional protection by RCD, one that instantly springs to mind in the 16th was a socket outlet for use of equipment outdoors, another was TT systems.

For as long as I can remember TT systems have utilized extra protection in either the old ELCB or the modern day RCD. All TN systems didn't until the 16th and the out side socket

The reason for this is the low EFLI path that a TN system gave you, and the stability of the DNO earth. The ESQCR empowered the DTi to make the DNO to supply stable earthing for consumers, and though the ranges are not actually in the guide, it was decided that a TN-S should be 0.80 and TNC-S 0.35.

If you found an installation that exceeded these values it would be an indication of a fault and as Tony said you really should not be masking a fault.

Yes an RCD would work in this instance, but you are not clearing the problem, which if ignored could mean a situation where the fault escalated to either a blown cable or leaving an installation without an earth at all. This is why we test.

I'm sure as you carry on working you will find a TN-S system that has an upfront RCD/ELCB and wonder what the.....................often what happened in TN-S times there would be fault/maintenance on a joint and the old Electricity board would fit an upfront RCD/ELCB as a temporary fix, and it would be just left.

 

[markthespark]

Thanks Malcolm, great post.