3 phase supply

[Robin Spark]

Small shop, three phase 100amp supply at cutout, however only 2 of the phases are being utilised.

The cutout and 2 consumer units are all located in a cupboard, however the tails from the cutouts are fed through the same galv trunking to each consumer unit.

Consumer unit 1 feeds downstairs ccts, Consumer unit 2 feeds upstairs ccts.

My question is can I treat this as a single phase supply when filling in the certificates as i need install a new radial cct for some socket outlets. IE would I need to follow rule of thumb and double my PFC readings?

 

[Andy™]

you should treat as TP.

also, depending on location, it may be split phase and not TP

 

[steptoe]

its most likely a 2 phase supply as that is exactly what you have described.

if it has one meter then it is one installation and must be treated as such,

a "normal" single phase installation cert is no good.

IMO.

---------- AUTO MERGE Post added at 22:19 ---------- Previous post was at 22:18 ----------

[quote name='Andy

 

[Andy™]

er... what is a 'normal' single phase installation cert?!

 

[steptoe]

[quote name='Andy

 

[Robin Spark]

Just to confirm It's a TNS with 3 *100amp fuses on cutout, each with a 25mm tail going to the meter its just that only 2 phases are being used, so surely 3ph supply.

So I will double the PFC on the EIC when I do the work, dont see much of an issue with PFC being high as I took a Zs reading at a nearby socket and got 0.56 ohms.

 

[kme]

I disagree with regards to the PFC.

If you only have single phase boards, you are incapable of having a phase-phase short there, so a phase-neutral PSC is fine.

Although the cert. should reflect that there is a TP supply, utilised as 2 x SP DBs.

KME

 

[sellers]

surely the meter and CU tails are more than capable of having a phase to phase short.

 

[kme]

Whether the meter is or isn`t capable shouldn`t concern you, TBH

Your PSC is taken at the DB - what I`m saying is that a single phase DB fed from 1 phase of a TP supply cannot be subject to a phase-phase short.

KME

 

[sellers]

I disagree KME, your DB PSC is taken at the board, but your supply PSC isn't and your supply PSC does concern you as with Ze and Zdb

 

[Robin Spark]

ok I agree with Kme that within each individual CU then PFC can be recorded as is, no need to double up result.

Also agree with Sellers that within the Galv trunking there is possibility of phase to phase faults, but that is on the tails and before each consumer unit.

However, you have now got me thinking that as both CU's are within the same cupboard then there could be a phase to phase fault on downstream ccts, but at the moment I cannot remember the exact routing of the cables for ccts from both CU's I will need to check this.

Also there arent any 400v warning labels so will put this on cupboard door and inside as well.

 

[kme]

I disagree KME, your DB PSC is taken at the board, but your supply PSC isn't and your supply PSC does concern you as with Ze and Zdb

Alright.

So can you explain where you measure "supply PSC" please?

Thanks

KME

 

[sellers]

well the pscc should be ideally taken at the cut out or by enquiry, twice the phase neutral psc as far as I'm aware

 

[spinlondon]

How many meters are there?

If there are two meters, then it is two single phase supplies.

If there is only one, then there is one three phase supply.

 

[sellers]

Sorry to disagree spin but you can have 1 3phase meter or 3 single phase meters on a 3 phase supply.

To me its as simple as this, do you have a 3phase service head, if yes you have a 3phase supply regardless of how many your using or how your using each phase!

 

[Andy™]

To me its as simple as this, do you have a 3phase service head, if yes you have a 3phase supply regardless of how many your using or how your using each phase!

but you dont know the service head is TP. it may be split, with L1 & L2 in use, and L3 spare.

 

[sellers]

You would because you would either pull the fuses and test or enquire?

Supply characteristics is exactly what it says on the tin, nothing to do with db's or meters!

In my opinion of course.

It kind of follows on from people who think each db needs its own entire PIR cert.

 

[steptoe]

so it could be 2 phase then?

having 3 fuses doesnt mean its 3 phase, one of the phases may not be connected,

not very common nowadays, but still to be seen in quite a few places.

 

[kme]

You would because you would either pull the fuses and test or enquire?Supply characteristics is exactly what it says on the tin, nothing to do with db's or meters!

In my opinion of course.

You are, of course, entitled to your opinion - you seem to suggest that you unseal the cutout, pull the main fuse, remove the cover from the cutout & do a PSC test there? On all your work?

It kind of follows on from people who think each db needs its own entire PIR cert.

EH?????

Look: lets get something simple & straightforward on the table here;

The supply cable, service head, meter tails and meter are completely off-limits to you, unless you work for the supply company or DNO!!!!

(You can advise relevant company of defects visually apparent with these items; nothing more.)

Consumers` tails (from meter to cutomers installation) are the FIRST parts of the install available to you.

Irrespective of there being one TP meter, or 3x SP meters - if the consumers tails go into different SP DB`s( without there being a TP isolator!), with no real possibility of 2 phases coming into contact, and the DB`s feed different areas which do not overlap - then, for all intent & purpose, you are dealing with an SP supply; and the closest point you can take a reading on PSC is the henleys( if fitted), isolator (if fitted) or distribution board(s).

My interpretation / opinion.

KME

 

[sellers]

Very true KME, but like I say or by enquiry, as it says in the regs.

But as a rule of thumb double your single phase psc. Every installation is different but to me I can see many ways 2 phases may short, be it if the tails get nipped in trunking, or a child takes something metal to the tails, anything is possible.