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Topic Title: Testing existing installations
Topic Summary: Testing existing installation when incoming supply cable increased
Created On: 18 February 2014 02:51 PM
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 18 February 2014 02:51 PM
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jonno

Posts: 29
Joined: 06 September 2006

An extension to a building meant that the existing main MCCB board (which was full) was replaced so that it could feed the existing DBs plus the new DBs installed in the extension. The total load has increased, so the DNO has installed a new larger feeder cable.

All the new installation in the extension has been EIC. Is any testing of the existing untouched installation required?

The Zs of the existing circuits must have changed, and probably for the better. So does this fact mean every existing circuit should be tested to record the new values? Also, should the short circuit capacity of existing CPDs be checked?

Regards
John
 18 February 2014 04:42 PM
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Banners

Posts: 117
Joined: 06 October 2013

If this is an installation under the supervision of maintenance staff who have records of EICRs etc then i would not have thought so. Even if there were no on site maintenance bod(s) but records of suitably spaced EICRs and other test results/certs then I would still say no. At the most, I would suggest a DB Ze is taken at each board and compared to previous known Ze. If (as you suspect) Ze is lower then common sense would suggest all is satisfactory.
 18 February 2014 04:48 PM
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John Peckham

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Joined: 23 April 2005

John


hopefully there is a an EIC for the new panel board with test results for the existing distribution circuits connected to the new board? I would make sure the existing earthing and bonding conductors are of sufficient CSA for the new supply and existing circuit protection is rated for the increased prospective fault current.

I would say the existing final circuits do not need re-testing assuming you have a current EIC or EICR for them in the 1st place?

Hope this helps

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John Peckham

http://www.astutetechnicalservices.co.uk/
 18 February 2014 05:02 PM
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Good point by Mr Peckham: reduced Ze = higher PFC = exploding (potentially) MCBs under fault conditions.
 19 February 2014 09:36 AM
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jonno

Posts: 29
Joined: 06 September 2006

Even if there is an existing EICR, the values for fault current and Zs will have now changed, due to the larger cable.
So even though the existing installation has not been physically changed, the electrical characteristics have been changed, so in the eyes of the Wiring Regs the installation has been modified and needs certifying?
The new parts of the installation have a new EIC.
So for the existing installation I would think that the new values should be recorded on a test sheet and appended to the new EIC or existing EICR?

Thanks for your thoughts

Regards
John
 19 February 2014 10:34 AM
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OMS

Posts: 19747
Joined: 23 March 2004

Originally posted by: jonno

Even if there is an existing EICR, the values for fault current and Zs will have now changed, due to the larger cable.

They will - but the designer will have considered that as part of the design to increase the supply capacity whilst retaining some parts of the existing installation.

So even though the existing installation has not been physically changed, the electrical characteristics have been changed, so in the eyes of the Wiring Regs the installation has been modified and needs certifying?

Not realy - BS 7671 doesn't require "re-certification" - the designer has the option to passs on the requirememt for any additional testing felt neccessary to ensure the installation remains compliant with BS 7671 or as a minimum fit for continued service.

The new parts of the installation have a new EIC.

Required by BS 7671 as a minimum

So for the existing installation I would think that the new values should be recorded on a test sheet and appended to the new EIC or existing EICR?

As I said, not a must. The designer will have signed the EIC for the new works and that EIC also contains a commentary on the existing installation and defines the extent of works covered.

Thanks for your thoughts

Personally, I would have determined at design stage that changing the incoming supply wasn't a problem - and I would have defined further points where I wanted testing (and what tests I wanted) within the installation to allow the installer and tester to verify my design assumptions.

Good engineering practice would suggest that those tests be recorded



Regards

John


OMS

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