TNS and 315a bs88

[pope]

Went to look at a job on Friday and I think I already have the answer just putting it out there.

Factory has been completely gutted been asked to put in a 315a TPN by the head running a sub main to the other part of the building they are going to be using.

My problem is I already have a Ze of .26 then when I add on the cable run it is going to be well over the .14 maximum zs allowed. Apart from putting in an rcd unit what (if any because I can't think of any) other options do I have?

Client is really not happy with the thought of rcd on the submain even tho I have explained it would be time delayed etc etc and they would never even know it was there.

 

[Bobster]

Went to look at a job on Friday and I think I already have the answer just putting it out there.

Factory has been completely gutted been asked to put in a 315a TPN by the head running a sub main to the other part of the building they are going to be using.

My problem is I already have a Ze of .26 then when I add on the cable run it is going to be well over the .14 maximum zs allowed. Apart from putting in an rcd unit what (if any because I can't think of any) other options do I have?

Client is really not happy with the thought of rcd on the submain even tho I have explained it would be time delayed etc etc and they would never even know it was there.

Not ideal, fit a 1A time delayed elcb an move on.

 

[Engineer54]

If you say this factory has been completely gutted, make sure all bonding cables are still in place and haven't ferreted away for scrap. So what was the Zs or Zdb value, because thats the value you need to work with not the Ze value

You say you are intending to run a 315A sub main, so what is the available supply from which you are taking this sub main from. Is there a factory TX on the premises??

 

[pope]

Bonding cables are still in place. Gas, water and structural steel what I could find and verify.

There is a 400a head and ct chamber.

I do not know the zs of the circuit yet as I am yet to install it. Only went there to have a look at the job and thought I should take some readings of the original install but quickly said it was a no go in using the old.

But then do not want to put in a 315a three phase fused isolator if I cannot then obtain the reading I require due to the Ze being above this already. Or am I missing something?

 

[Engineer54]

Well YES, you should be measuring The Zs value in relation to OCPD for a start, NOT the Ze value!!

 

[pope]

Care to explain a bit more fella. How can I take a zs of the circuit if there is no circuit there for me to test? I don't want to go putting in a circuit that does not comply? I don't see how I can reach my .14 zs at the db for the OCPD if the Ze is already above this? Tried taking a reading at the ct chamber one with all bonding connected and one just using the suppliers earth.

 

[Engineer54]

I suspect you may have already measured the Zs/Zdb at the origin on this installation... I take it you did disconnect the main earthing conductor from the MET when you supposedly tested the Ze value?? If you did, then you need to now test with the main earthing conductor connected to the MET which will give you a vastly different value from a Ze value!!

BTW, there are many ways to bring Zs values down, the last resort being relying on an earth leakage device.

 

[pope]

Yes I tested Ze with main earth disconnected from side of the head.

Then tested zs/zdb at origin of supply from the MET but the changes in reading where negligible still over my maximum zs allowed for the circuit i was looking at installing. So no way with the extra cable run all be it only around 10m would I be able to reach the zs required.

Yep I completely understand relying on an earth leakage device should be last resort so what other ways can I bring down the Zs values I've never had this issue before so thought I'd ask over here for a bit of help.

 

[Engineer54]

Yes I tested Ze with main earth disconnected from side of the head.

Then tested zs/zdb at origin of supply from the MET but the changes in reading where negligible still over my maximum zs allowed for the circuit i was looking at installing. So no way with the extra cable run all be it only around 10m would I be able to reach the zs required.

Yep I completely understand relying on an earth leakage device should be last resort so what other ways can I bring down the Zs values I've never had this issue before so thought I'd ask over here for a bit of help.

I can't see that to be honest, especially if extraneous structural steel is making up part of that Zs value. Only ever seen a Zs value being of negligible difference where there has been little to zero extraneous earthing being present... Remember the 315A OCPD (fuses) will be located at the origin of the supply close to the cut out, not at the DB 10 meters away. The OCPD's contained in that Board will be of a significantly lower current rating than 315A and therefore a significantly higher Zs value being acceptable for ADS compliance

Some of the older service metal cut out housings were used as a means of providing an earth point, if that's the case, take a suitably sized conductor direct from the PILC cable sheath using a wrap around constant spring pressure clamp to the MET. Providing supplementary earthing and earthing paths, Verify that any other metalwork within the factory is extraneous and if found to be, connect to the MET...

It may well come down to using a form of Earth Leakage device, but it won't be your typical 300mA to 30mA S type variety, more like a CT based earth leakage relay with adjustable trip and time elements...

 

[pope]

Yep I get that completely. It is only a small factory so not a lot in the way of extraneous metal work so this would be why my readings are so close.

The cable that comes in is not possible to use that sort of clamp as it is not lead sheathed it is PVC and so no practicable to use that method. Cheers for the idea tho if I do come across that set up one for later use if required on another install

Yep I know what you mean I have installed the type below on several projects.

Just as soon as I start to mention rcds his back goes up straight away as he has had problems with them in the past. Maybe it might be worth putting him on the phone to other clients I have used this method with before with no problems what so ever.

 

[JameZZZ]

Could you swap the BS88 for an adjustable electronic MCCB (mircologic or something similar) which has more sensitive trip characteristics and a higher allowable Zs. You would have to ensure discrimination with downstream devices.

 

[ImpededLoop]

Firstly, it's refreshing to see a well thought out job.

Just a thought, have you or the customer tried contacting the DNO in case they have TNCS available? It may enable you to get a much lower Ze.

Also, who had specified 315A? Would it be possible to reduce this to give a bit more headroom for the Zs?

 

[ImpededLoop]

Or another thought, would you be able to run two or three submains at 100A if that gets you within the required Zs?

 

[pope]

I don't like running into jobs chucking something in then thinking about things like this as an after thought rather be clued up before I start the job.

Every time I've put in anything like this the trusty.

With spreader box then out from there.

Has been my go to method of install but does not look like this will be the case with this install. For a client I do quite a bit of work for so wanted all my facts straight before I talk to them about what is required.

Three separate sub mains is a no go as they want everything in one db and makes a lot more work. I have the keys so might go do some investigation and see what I come up with I know I have the job already but just want the best for the client.

 

[davesparks]

But they might compromise at 2x DB next to each other if it avoids then having the RCD they obviously don't want

 

[Engineer54]

But they might compromise at 2x DB next to each other if it avoids then having the RCD they obviously don't want

But the switch fuse will still be at the origin of the installation and this is where the problem is, the final DB MCB's shouldn't have a problem with the Zs values being quoted above...

 

[davesparks]

But the switch fuse will still be at the origin of the installation and this is where the problem is, the final DB MCB's shouldn't have a problem with the Zs values being quoted above...

But with 2xDB at the far end he could run 2x smaller sub mains which might have a more achieveable max Zs

 

[Engineer54]

Were now getting into the realms of over design just to try and cater for a higher than expected Zs. So now you'll need a suitably sized front end isolator switch, rated at say 300A, Two suitably sized switch fuses rated at say 125/150A, and two suitably sized DB's......

 

[ImpededLoop]

Were now getting into the realms of over design just to try and cater for a higher than expected Zs. So now you'll need a suitably sized front end isolator switch, rated at say 300A, Two suitably sized switch fuses rated at say 125/150A, and two suitably sized DB's......

A lot depends on the layout of the building. If positioning two distribution boards in different places reduces the length of the final circuits, it may be advantageous.

 

[Engineer54]

A lot depends on the layout of the building. If positioning two distribution boards in different places reduces the length of the final circuits, it may be advantageous.

It's not what happening here though is it?? The planned position of the DB is stated to be 10 metre's away...