Earthing...... Cable size any help appreciated

[lippy]

I have recently taken over responsibility for a care home which has a 3ph supply,
Incoming phase cables are 185mm, fuses are rated at 315A and the earth is 50mm, PFC is 1.15kA x 2 and the Ze is 0.20 ohms

I have been given a PIR which was carried out last year, stating amongst other things that the Earth should be 100mm based on the calculation that the earth CSA should be Phase csa /2 ( reg 543.1.1)

If I have worked it out correctly there is another calculation which is;-

S (csa of earth cable ) = Sqrt of I squared x t / K ( 143 for copper cables)

so;-
I = 2.3kA
t = 5 secs
K= 143

sqrt of (2300 x 2300) x 5 / 143

my calcs say S =35.96mm ????? so the 50mm in place should be sufficient ??????

its many years since I've done this stuff, can anyone help and check my calcs ? or am I missing something like a life????


Regards Ian

 

[D Skelton]

I'd say you're nearly there with your calculation.

't' isn't five seconds though. 't' is the time in which the manufacturer states the OCPD will blow/trip given the prospective earth fault current. You will need to look at the time/current graphs provided by the manufacturer of the OCPD in question. The time/current graphs in BS 7671 can also be used but will not be as accurate. Also, your PEFC is 1.15kA, not 2.30kA.

Two things though.

1. You never stated what type of earthing arrangement is present? If PME, what I've just said goes out the window. Size the Earth according to the required size of bonding conductor.
2. 1.15kA seems very low for a 300A supply.

 

[lippy]

sorry,
I am sure its a PME supply, the reason I used 1.15 KA is because I am going off the PIR report and i believe that its normal to double the PFC for 3 phase

Cheers Ian

 

[Deleted member 26818]

With 3 phase, it is normal to double the highest measured value for PSCC, and use that value for PFC.
Once calculated and recorded, there would be no necessity to double the figure again.

 

[Anthony S]

Rule of thumb to double PSCC on 3 Phase

 

[lippy]

Which is what I have done assuming that the recorded PFC on the PIR is correct and for one phase, as it is marked on the report as '1.15kA x2'

 

[D Skelton]

sorry,
I am sure its a PME supply, the reason I used 1.15 KA is because I am going off the PIR report and i believe that its normal to double the PFC for 3 phase

Cheers Ian

In my experience, unless you know who filled out a previous EICR or you can at least read through it and get a general idea that the person who filled it out has half a clue, I'd treat it as nothing but glorified bog roll. As for the installation being PME, refer back to my later first point in my previous post.

 

[D Skelton]

With 3 phase, it is normal to double the highest measured value for PSCC, and use that value for PFC.

Rule of thumb to double PSCC on 3 Phase

Which is what I have done assuming that the recorded PFC on the PIR is correct and for one phase, as it is marked on the report as '1.15kA x2'

If working out the size of an earthing conductor you use PEFC for this.

A rule of thumb for working out PSSC on a three phase system is to double the L-N PSSC. But why would you be using the prospective short circuit current to work out the size of an earthing conductor???

 

[lippy]

Thanks, but re the 't' being the time taken for the fuse (OCPD) to blow, then surely that would be the maximum for that type of circuit which is 5 seconds ???

 

[lippy]

If working out the size of an earthing conductor you use PEFC for this.

A rule of thumb for working out PSSC on a three phase system is to double the L-N PSSC. But why would you be using the prospective short circuit current to work out the size of an earthing conductor???

Yeas I see what you mean, but isnt the PEFC and PSSC almost the same ?????

 

[D Skelton]

Thanks, but re the 't' being the time taken for the fuse (OCPD) to blow, then surely that would be the maximum for that type of circuit which is 5 seconds ???

5 seconds is the maximum time allowed, but it won't be the actual time it takes.

 

[Anthony S]

The PSSC on 3 Phase will be higher than the PFC so you can't be wrong if using the figure if that's all you have to go on ? or am I wrong in thinking that ?

 

[davesparks]

Yeas I see what you mean, but isnt the PEFC and PSSC almost the same ?????

Nope, one is the maximum current which will flow through a direct fault from live to earth, the other is the maximum current which will flow through a direct fault between live conductors.

They may well be similar or identical on a single phase supply, but won't be on a three phase supply.

 

[lippy]

looking thru my regs now for curves but cant find BS88 HRC on the list

 

[lippy]

Nope, one is the maximum current which will flow through a direct fault from live to earth, the other is the maximum current which will flow through a direct fault between live conductors.

They may well be similar or identical on a single phase supply, but won't be on a three phase supply.[/QUOTE

Ah Gotcha ... thanks

 

[Anthony S]

looking thru my regs now for curves but cant find BS88 HRC on the list

yellow book page 324 I believe then your see how close you are

 

[D Skelton]

The PSSC on 3 Phase will be higher than the PFC so you can't be wrong if using the figure if that's all you have to go on ? or am I wrong in thinking that ?

Yes, very wrong. Short circuit current will not flow through an earthing conductor!

 

[Deleted member 26818]

I think things are getting a bit bogged down with half explanations.
When determining the minimum CSA of an earthing conductor for a 3 phase installation, you would measure the PEFC for each phase, then use the highest measured value in your calculations.
Where PME conditions apply, the minimum CSA should be determined from the appropriate table in BS7671, based on the CSA of the line conductors.
Where (as appears to be the case in this instance) an EICR is being conducted, it usual to first ensure by calculation that the CSA is adequate for fault protection, then to examine the conductors for signs of over heating.
If the CSA is adequate for fault protection, but is undersized as per the table in BS7671, it generally accepted that no upgrading is required, as long as there are no signs of heat damage to the earthing/bonding conductors.

BS88 fuses are HRC.

 

[D Skelton]

I think things are getting a bit bogged down with half explanations.
When determining the minimum CSA of an earthing conductor for a 3 phase installation, you would measure the PEFC for each phase, then use the highest measured value in your calculations.
Where PME conditions apply, the minimum CSA should be determined from the appropriate table in BS7671, based on the CSA of the line conductors.
Where (as appears to be the case in this instance) an EICR is being conducted, it usual to first ensure by calculation that the CSA is adequate for fault protection, then to examine the conductors for signs of over heating.
If the CSA is adequate for fault protection, but is undersized as per the table in BS7671, it generally accepted that no upgrading is required, as long as there are no signs of heat damage to the earthing/bonding conductors.

BS88 fuses are HRC.

Agreed.

Things are also getting bogged down however with people chipping in with "Double your PFC to get the true PFC" when we are talking about sizing earthing conductors. I think it's also getting bogged down with people who don't quite know the difference between PEFC and PSSC.

Back to basics is a suggestion I'd be inclined to make!

 

[lippy]

yellow book page 324 I believe then your see how close you are

i only have the green one !!!!