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

[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 !!!!

 

[Anthony S]

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!

Who said double your PSSC to get the true PFC ?

 

[D Skelton]

Who said double your PSSC to get the true PFC ?

I was paraphrasing. See post #8

 

[lippy]

I think one of the original posts says it all, I am taking data from a PIR or EICR which might as well be treated as bog paper......

I will go back and do some looking and testing myself next week. however.......... is the original calc ok assuming I alter 't' ???

 

[D Skelton]

I think one of the original posts says it all, I am taking data from a PIR or EICR which might as well be treated as bog paper......

I will go back and do some looking and testing myself next week. however.......... is the original calc ok assuming I alter 't' ???

And as long as you use the correct value for 'I', which is the prospective earth fault current, the highest measured value between one of the line conductors and earth.

 

[Anthony S]

Just re-read the op and realise I've been quoting pssc when it says clearly says pfc. Gods knows what I'm on.

 

[lippy]

Gents thanks for your help, I will update when I have done my own testing

Cheers

 

[Deleted member 26818]

I have a feeling that you are going to be right on the edge of the 5s disconnection time with that PFC and rating of fuse. If the PEFC is any lower, you may have trouble satisfying the 5s disconnection time.

 

[Geordie Spark]

Just re-read the op and realise I've been quoting pssc when it says clearly says pfc. Gods knows what I'm on.

Whatever it is you're on pass some my way ......... please!! :sifone:

 

[Engineer54]

I think one of the original posts says it all, I am taking data from a PIR or EICR which might as well be treated as bog paper......

I will go back and do some looking and testing myself next week. however.......... is the original calc ok assuming I alter 't' ???

Don't worry too much about the DNO fuses, they are basically no concern of your's, the DNO use their own parameters for the protection of their network distribution cables. Any protection that is on the consumers side of this installation will be based on the manufacturers OCPD minimum ''Zs'' requirements, not the Ze value, although the Ze value at the source will play a significant part of the installations Zs/Zdb values...


So when you go back to do your'e own tests, check both Ze and Zs....

 

[OhmElectrical]

Have you done adibiatic ?