Loop impedance test

[mpelec]

Hopefully someone can answer this query for me.

I'm more used to domestic installations but have taken on a job in a workshop. The workshop is around 80m away from the intake which is a 3 phase TN-C-S supply. The workshop is new but I'm using an existing 16mm 4 core SWA cable. I'm using the SWA as the earth. The ZE at the origin is within limits but when measured on the workshop DB I get 0.70 ohms, I'm guessing due to the length of cable & that I'm using steel rather than a copper conductor.

When measuring ZS at the sockets etc I'm getting a higher reading than those allowed in table 4.1.2 of the regs for type c rcbo's. If I changed the rcbo's for type b's the readings would be within limits but I'm worried they would trip with an inrush of current.

Any ideas? Thanks, mpelec.

 

[Marvo]

The ZE at the origin is within limits but when measured on the workshop DB I get 0.70 ohms, I'm guessing due to the length of cable & that I'm using steel rather than a copper conductor.

What is you ZE testing at origin?

 

[Darkwood]

What size fusing is upstream of this SWA ... Volts drop would be a concern too TBH!..

16mm at 80m dont go far re' ccc after calcs

 

[Sparkie30]

As Darkwood has said volt drop is major problem, even if you have had 16mm earth you will still have a problem due to length of it. i think you might need a 35 mm cpc 25 mm mate.

 

[D Skelton]

Hopefully someone can answer this query for me.

I'm more used to domestic installations but have taken on a job in a workshop. The workshop is around 80m away from the intake which is a 3 phase TN-C-S supply. The workshop is new but I'm using an existing 16mm 4 core SWA cable.

What fuse size are you using? I'm going to guess 63A. Your volt drop will almost certainly be an issue if this is the case.

I'm using the SWA as the earth. The ZE at the origin is within limits but when measured on the workshop DB I get 0.70 ohms, I'm guessing due to the length of cable & that I'm using steel rather than a copper conductor.

You're guessing?? You should know.

When measuring ZS at the sockets etc I'm getting a higher reading than those allowed in table 4.1.2 of the regs for type c rcbo's.

Please tell us the maximum Zs for a circuit protected by a 30mA RCD or RCBO on a TN system. You have some REAL problems if this is being exceeded!

If I changed the rcbo's for type b's the readings would be within limits but I'm worried they would trip with an inrush of current.

Inrush? On socket outlets?

Any ideas? Thanks, mpelec.

I have an idea. You're probably better off sticking with domestic stuff fella. You're way out of your depth here.

 

[Engineer54]

Agree with the other posters!!

How is the existing 16mm 4 core swa installed?? If it's installed in a large enough duct you may be able to pull in another 16mm 4 core SWA cable, but to be honest over 80m, i'm doubtful even if it's a straight run. If it's direct buried, again you could excavate the existing trench and run in a second cable. DO NOT try and run in a second cable if you can't follow the existing cable run, the two cables need to be of the same length in order to comply with a parallel cable supply. Whatever you end up doing to supply this workshop, you'll need to include a suitably sized separate earth/bonding conductor, as i'd lay money on there being extraneous metalwork being present within the workshop....

 

[DRG]

Is the csa of the armourings large enough to be used as main protective earth to remote DB anyway with a tncs supply?

 

[Darkwood]

De'Ja' Vous..... New member asks a question ..we ask for more info and detail and 19hrs later no sign of OP ... why we bother to give our advice for free sometimes runs around my head... its ignorant and takes the urine out of our good nature!

 

[Engineer54]

Is the csa of the armourings large enough to be used as main protective earth to remote DB anyway with a tncs supply?

Calculate it, using the CSA of the SWA, it'll be no different to a TN-S supply. It certainly won't be suitable for any bonding purposes!!

 

[Spoon]

De'Ja' Vous..... New member asks a question ..we ask for more info and detail and 19hrs later no sign of OP ... why we bother to give our advice for free sometimes runs around my head... its ignorant and takes the urine out of our good nature!

We do it cause we like helping, and the majority of the OP's are not pig ignorant..... At least I hope not.

 

[pitkin2020]

Or maybe the mods haven't approved further posts yet??

 

[Spoon]

Or maybe the mods haven't approved further posts yet??

Thats what I initially thought, but as darkwood has pointed out, its been 19 hours. I hope that the mods would have reviewed all the OP's posts by then.

 

[Marvo]

There's nothing in the moderation queue, what you see is all there is I'm afraid.