Installaton Test Cert.

eagleeye · Aug 27, 2009

I have just tried to fill out my first cert for a new rewire. i know i should know this but am stuck and hope someone can help. The rewire was a TT system and i used a Legrand CCU with RCBO's for every circuit. right, the form....
Im registered with the NICEIC as a Domestic Installer just so you know what forms im on about

The electrode resistance, Ra. All i did was measure the earth fault loop impeadence which was14.5 ohms. what should this be ?

next the method of measurement for earth electrode ?

"the nature of supply parameters" part. external earth fault loop impedance Ze and the part "particulars of installation ath the orgin - measured Ze. are these the same or do i leave one blank ?

"characteristics of primary supply overcurrent protective device/s" - the BS(EN) number of main fuse. is this BS 88 ? and what type should it be ?

should maximum demand be about 40 amps ?

sorry there is quite a few things but i hope you can help

zupos40 · Aug 28, 2009

eagleeye said:
I have just tried to fill out my first cert for a new rewire. i know i should know this but am stuck and hope someone can help. The rewire was a TT system and i used a Legrand CCU with RCBO's for every circuit. right, the form....
Im registered with the NICEIC as a Domestic Installer just so you know what forms im on about

The electrode resistance, Ra. All i did was measure the earth fault loop impeadence which was14.5 ohms. what should this be ?

next the method of measurement for earth electrode ?

"the nature of supply parameters" part. external earth fault loop impedance Ze and the part "particulars of installation ath the orgin - measured Ze. are these the same or do i leave one blank ?

"characteristics of primary supply overcurrent protective device/s" - the BS(EN) number of main fuse. is this BS 88 ? and what type should it be ?

should maximum demand be about 40 amps ?

sorry there is quite a few things but i hope you can help

1. Maximum demand for domestic installation I do not bother estimating it now unless there 2 electric cookers or 2 showers or supplies to external building. For a standard 2 or 3 bed house I assume the following if it got some form of central heating gas or oil 60A Max, if no central heating I assume 100A.

2. The main fuse is usually either a 63A or 100A BS1361 type II

3. The Ze is Ra in this case, you can measure Ze using the alterative method as discribed in GN.3.

Inform client that you will be doing test (ensure persons and animals, livestock are no where near earth electrode)
Switch off main switch, lock off etc, test it is isolated etc, labeletc, I also switch off all rcds and mcbs or remove fuses.
Remove main earth conductor from MET
you are now ready to perform Ze test as you would for TN earthing systems
I would reconnect earth repeat test to see if Ze goes down due to parallel paths, then measure Pefc and Pscc before removing lock and switching on supply

if that how you did your test and got 14.5 ohms then that your Ze and the high current reading of either Pefc or Pscc should be recorded as PFC for installation.

4. Not NICEIC but would put method as alternative method as discribed in GN.3

sixfingers · Aug 29, 2009

zupos40 said:
2. The main fuse is usually either a 63A or 100A BS1361 type II

63 A ? 60 A
You may also find an 80 A fitted.

Spudnik · Aug 30, 2009

Or a BS88

NoSparks · Aug 31, 2009

The whole point is you cant say what a cut out is unless
1) Its marked on the fuse carrier
2) you inspect it (which of cours you are not allowed to).

So if its not marked and you can't inspect it you say NO Access on the form.

Does it matter.
Well YES, if the Max Load is more than say 1.5 times the rated current of the cutout then it needs further investigation.

Which brings us to Maximum Load for which there are tables in the OSG.
I've not found 2 cookers yet, but two showers is quite common. That's 90A or 100A before you look at the rest!

I've not looked up the figures recently, but a BS1361 (which superceeded the BS88) will take double current for a short period of time and 1.5 x rated current for a considerable period.

If you'r over the limit then what is the pattern of use? How often will Max Load be encountered and for how long? What will be the voltage drop on the service cable? Will any equipment fail because of the voltage drop? Will the service cable melt because of the overload (I've seen that) and what do you do. Most domestic customers could not afford a service upgrade but you would need to discuss the situation with them.