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Topic Title: Installation Certificate query
Topic Summary: PME or TT
Created On: 08 November 2017 06:13 PM
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 08 November 2017 06:13 PM
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radiopotty

Posts: 10
Joined: 24 May 2016

I have a metered supply to a small building site. The incomer is BS1361 PME, From the meter I have installed a uPVC din rail enclosure containing a 100mA RCD and a 63 amp Type C 60898 mcb..An earth electrode is provided at this point.

My question is . Is the supply characteristics supply for the purposes of section 1 of the Installation Certificate the PME and its Ze and PSC ( incidentally .0.31 ohms. Or is it the Ze and PSC of the earth electrode. (incidentally) 22.6 ohms.
And what is the tick box reference. TNC or TT. ?
Many thanks.
Radiopotty
 08 November 2017 06:54 PM
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KFH

Posts: 554
Joined: 06 November 2010

I would say it depends on which earth you are using for the installation. So if you have TTed the installation and are not using the supplier PME then I would record it as TT.
 08 November 2017 07:51 PM
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alancapon

Posts: 6914
Joined: 27 December 2005

Yes, it is TT as you are using the separate electrode for the earth. This would be correct for a building site, as that should not use the PME earth.

Regards,

Alan.
 08 November 2017 08:19 PM
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mapj1

Posts: 9553
Joined: 22 July 2004

Is the earth for the installation from rod the only, then it is TT. Expect Zs to measure 50 ohms or so.

If the earth for the installation is from the rod in parallel with the company PME terminal, then it is PME, just with a piece of earthed metal in parallel. Expect Zs to read less than on ohm.

-------------------------
regards Mike
 09 November 2017 03:55 PM
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radiopotty

Posts: 10
Joined: 24 May 2016

Many thanks for your help with this.
So completing section 9 Supply Characteristics ; Earthing Arrangement is TT. External earth fault impedance will be the electrode resistance and Supply protective device is the 60898 mcb. Have a Nicky inspection shortly and he can be prickly.
 09 November 2017 05:41 PM
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mapj1

Posts: 9553
Joined: 22 July 2004

Note that your design needs to stand both values of PSSC - for LE faults and L-N ones so the worst case is the L-N case this time.

-------------------------
regards Mike
 09 November 2017 06:32 PM
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sparkingchip

Posts: 10094
Joined: 18 January 2003

A little clarification.

Presumably you have connected the site installation main earth terminal to the earth electrode and the DNO PME earth terminal has not been used, so the installation is TT direct earthed.

So you need to do separate tests to get the required test results and then record them on the certificate.

First connect the tester live lead to the incoming live and the earth lead to the earth electrode and do a earth loop test. The result Ra is recorded as the external loop impedance Ze and must be below the maximum of 500 ohms for use with an 100 mA RCD, ideally it should be below 100 ohms as a higher result may be unstable.

Secondly connect the tester live lead to the incoming live and the earth lead to the incoming neutral and do the prospective short circuit current test. The result is recorded as the prospective fault current Ipf and generally it seems to be between 0.7 and 1.6 Kva on single phase supplies to houses etc, it should not exceed the rating of the consumer unit and the devices it contains that have been installed.

Don't record the Ipf of the earth electrode or your assessor will get very picky!!

Andy B.

Edit- I have assumed you have used an dedicated earth loop tester such as the Megger LT320 that requires you to do an earth loop test then calculates the Ipf for you from that result without a further test by pressing a button.

Edited: 09 November 2017 at 06:42 PM by sparkingchip
 10 November 2017 11:09 AM
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AJJewsbury

Posts: 15990
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The result is recorded as the prospective fault current Ipf .... it should not exceed the rating of the consumer unit and the devices it contains that have been installed.

Although for UK style consumer units (i.e. complying with annexe ZB of BS EN 61439-3 or older equivalents) the overall CU may be taken as being rated at 16kA regardless of the rating of the individual protective devices it contains.

- Andy.
 10 November 2017 01:13 PM
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sparkingchip

Posts: 10094
Joined: 18 January 2003

Originally posted by: AJJewsbury

The result is recorded as the prospective fault current Ipf .... it should not exceed the rating of the consumer unit and the devices it contains that have been installed.


Although for UK style consumer units (i.e. complying with annexe ZB of BS EN 61439-3 or older equivalents) the overall CU may be taken as being rated at 16kA regardless of the rating of the individual protective devices it contains.



- Andy.


Because it is assumed the suppliers fuse will limit the damage.
 11 November 2017 10:55 AM
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AJJewsbury

Posts: 15990
Joined: 13 August 2003

Because it is assumed the suppliers fuse will limit the damage.

Or that a fault is unlikely to occur in the first 3m of cables after the CU, or that any products of explosion/arcing will be contained within the outer casing.

Given the detail we usually have to design to it's sometimes surprising what 'hopeful assumptions' are made at a higher level...

- Andy.
 13 November 2017 07:48 AM
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gkenyon

Posts: 4954
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Originally posted by: AJJewsbury

The result is recorded as the prospective fault current Ipf .... it should not exceed the rating of the consumer unit and the devices it contains that have been installed.


Although for UK style consumer units (i.e. complying with annexe ZB of BS EN 61439-3 or older equivalents) the overall CU may be taken as being rated at 16kA regardless of the rating of the individual protective devices it contains.



- Andy.
More correctly, for UK style assemblies that are type-tested in accordance with Annex ZB ... which therefore is not any old enclosure (to the standards), plus any old devices to the standards ... it in reality means the manufacturer's enclosure AND the devices from the SAME RANGE that have been type-tested together.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
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