Exceeding max zs values on caravan site? With pme supply

[Dean7247]

Hi all, first post on the forum and I hope you's can help me with a few questions I have.
I have been asked to test a caravan site with the following details
pme supply (with no individual spikes at hook up points)
main 400a mccb with bolt on adjustable rcd(turned in the off position)i.
I have the previous paperwork from 3 years ago but it states that the supply is tns but on the end box it says pme.
Many of the submain circuits exceed the max zs reading stated by bs7671.
My main questions are
1. Is the main mccb with bolt on rcd to provide the correct disconnection times on the circuits with high zs readings
2. Is the only way to fix the pme side of it to drive spikes at each individual hook 1up point.
If any of you's could shed a bit of light on this I would be grateful , thanks Dean

 

[Sintra]

Hi Dean, firstly I think you need to provide a lot more information about this set up as I've read it a few times and can't get what you're trying to say. Secondly I would advise that you read and understand section 7 of bs7671.

 

[Engineer54]

Well for a kick off, PME is NOT allowed to supply caravan/mobile home plots, and you should have already been aware of that fact before taking this work on!! So the first thing you need to ascertain is whether this ''IS'' a DNO's PME supply or a TN-S supply.

Can't give you anymore advice until you come back with a definite answer to the above question...

EDIT.....

It could well be that the TN-S supply has been PME'd during that 3 year period since it was last inspected/tested. But then i find it hard to believe that the DNO would have carried out the conversion on such a business installation without first insisting that the installations supplying the caravan plots had been TT'd at the caravan take off positions.

 

[spark 68]

It can be supplied initially from a PME supply at the origin, However this MUST then be converted to a TT supply (by segregating the PME Earthing and adding Earth Electrodes) BEFORE feeding the pitches, this need not necessarily be done at each Pitch, nor need a rod at each pitch, it could be done at distribution level OR at each/or group of pitch/es.

There are several methods in which this can be achieved, the size of the pitch and number of points served will have a bearing on the most economic method that is used.

I would suggest you get a copy of GN7 if you are going to take this type of work on, although everything is in BS7671 section 708, GN7 explains it a lot better and clearer and actually shows how to implement the regs to achieve compliance.

A TNS or TT supply at the origin can be supplied to each pitch subject to the rest of that section being complied with.

 

[Dean7247]

I have not actually tested anything yet but just run my eye over it. I have read section 7 of the regs and understand that a pme system is only permitted to the permanent/fixed buildings but not the caravans themselves.
I will be enquiring further on what type of system it is as a label on the end box would no suffice the massive undertaking of driving 252 spikes and pits at each hook up point without a definite answer. The real question was the high zs readings and what exactly the main mccb with bolt in adjustable rcd was for?
The set up goes roughly like this

main dno fuses to
main 400a mccb with adjustable rcd to
submain fdu boxes and splitters located around park to
individual caravan hook up points incorporating 30mA rcd and mcb.

Also if the spikes do need installed what is the zs reading that is filled out on cert, is it the pme earth going to each hook up point which is not connected to the caravan or is it the spike reading or both?

 

[spark 68]

As I already said above, depending on how large the site is and how many pitches, it can be arranged a few different ways, GN7 explains all far clearer than either the regs book or I can on an internet forum.

A search on this subject on here may show some pages from GN7 that I posted up a while back, do yourself a big favour and get the book if you are doing this type of work.

 

[Engineer54]

First and foremost find out for certain which earthing system is actually being supplied by the DNO.

What do you mean by the term ''MCCB with Bolt on RCD''?? Do you mean a MCCB incorporating adjustable parameters, ground fault being one of them (normally rating and time), because i've never seen an MCCB with a bolt on RCD??

As has been stated there are many ways to distribute power to caravan patches around the site, if installed with several area distribution boards the PME needs to be isolated at these DB's and a TT system created from then on. Or leave PME in place at the area DB's and TT at the multi (usually twin) patch take off points...

 

[JD6400]

First and foremost find out for certain which earthing system is actually being supplied by the DNO.

What do you mean by the term ''MCCB with Bolt on RCD''?? Do you mean a MCCB incorporating adjustable parameters, ground fault being one of them (normally rating and time), because i've never seen an MCCB with a bolt on RCD??

I think the OP is referring to a earth fault block and possibly a combined shunt trip ( depending on frame size ) We have used them from several different makers in the past . I know Hager list them and Proteus , and I am pretty sure Shcnider do as well .
I generally come across them on gensets supplied to run mobile driers

 

[Engineer54]

I think the OP is referring to a earth fault block and possibly a combined shunt trip ( depending on frame size ) We have used them from several different makers in the past . I know Hager list them and Proteus , and I am pretty sure Shcnider do as well .
I generally come across them on gensets supplied to run mobile driers

DO you have a link top these Earth Fault blocks??

 

[JD6400]

I am using a new notebook , as my lap top has at last given up !
I also have fewer computer skills than an average farm yard animal , so the best I can suggest is to Google " mccb shunt rcd " and I think the Hager PDF is the second one down .

 

[JD6400]

I think the Google link says it is a untitled PDF , but when you down load it it comes up as " Protection Devices " , I hope that is of help ?

 

[Engineer54]

I think the Google link says it is a untitled PDF , but when you down load it it comes up as " Protection Devices " , I hope that is of help ?

Ok i found what i think you are talking about.

They look to be a sort of retrospect device, to upgrade a MCCB that doesn't incorporate GF protection. Never used them myself, Obviously on new projects if you require GF protection at the origin of the the installation, that protection would be incorporated in the specified OCPD(MCCB/ACB)

A replacement 400A MCCB incorporating such protection would cost considerably more than adding one of these retrofit devices, so i can understand the thinking behind providing such a module!!

 

[richy3333]

Hi all, first post on the forum and I hope you's can help me with a few questions I have.
I have been asked to test a caravan site with the following details
pme supply (with no individual spikes at hook up points)
main 400a mccb with bolt on adjustable rcd(turned in the off position)i.
I have the previous paperwork from 3 years ago but it states that the supply is tns but on the end box it says pme.
Many of the submain circuits exceed the max zs reading stated by bs7671.
My main questions are
1. Is the main mccb with bolt on rcd to provide the correct disconnection times on the circuits with high zs readings
2. Is the only way to fix the pme side of it to drive spikes at each individual hook 1up point.
If any of you's could shed a bit of light on this I would be grateful , thanks Dean

Is this C&CC work as it sounds like it from the description and the time of year?