Testing question following successful 2391 result

p11jor · Jul 30, 2012

Hi all I passed my 2391 about 6months ago and thought I was flying, I know test procedures & how to carry out the tests like the back of my hand and I fully understand any results I get and how to rectify or diagnose any faults in a electrical system.....but then again I have only tested ddomestic installations.

We have recently been doing some wiring on a site, 55 flats & 3 shops below, we're would you even start on this!? (I am not testing it by the way before anyone jumps on me) it's been tested a few weeks back but was so busy on there I couldn't get to put my eyes over what they did.

The apartments had a 10mm swa from the plant to each flat, so I understand Would have to take a ZDB for the swa ze + (r1+r2) of swa
the apartments are a doddle
were would the Zdb reading be noted? Would the supply cable be noted on the schedule of results for the appropriate apartment but marked as a supply cable instead of a final?

Or would it be marked down on the test sheet for the plant room, again what tests would be carried out in the plant room with all the kwh meters, the 125mm armour coming in would they take a Ze from this? How would the pfc work, would you go from all three phases and record highest?

I really don't have the knowledge or experience I thought from the 2391 and this was certainly not covered!

Cheers guys

Guest123 · Jul 30, 2012

The main panel board in the plantroom which we'll call DB-1 will have it's own schedule of inspections, circuit details and test results. Each of the distribution circuits would be recorded on these schedules with their I/R, R1+R2, Zs etc.

As this is the origin the "details of the installation at the origin" would also be recorded here i.e, suppliers OCPD's, supply characteridtics, earthing system etc. Ze & PFC would also be recorded here, the PFC is the highest value of the PEFC (earth fault current) or PSCC (L-N short circuit current) which on a TN-C-S system 'should' be the same.

In a 3P system however the PSCC (L-N short circuit current) is doubled (technically it should be multiplied by 1.732 but doubleing errs on the side of safety) (unless your instrument can measure between phases), as the highest fault current would normally occur between phases.

Farmelectrics · Jul 30, 2012

10mm supply to each flat did you say

johnboy6083 · Jul 30, 2012

Lenny said:
The main panel board in the plantroom which we'll call DB-1 will have it's own schedule of inspections, circuit details and test results. Each of the distribution circuits would be recorded on these schedules with their I/R, R1+R2, Zs etc.

As this is the origin the "details of the installation at the origin" would also be recorded here i.e, suppliers OCPD's, supply characteridtics, earthing system etc. Ze & PFC would also be recorded here, the PFC is the highest value of the PEFC (earth fault current) or PSCC (L-N short circuit current) which on a TN-C-S system 'should' be the same.

In a 3P system however the PSCC (L-N short circuit current) is doubled (technically it should be multiplied by 1.732 but doubleing errs on the side of safety) (unless your instrument can measure between phases), as the highest fault current would normally occur between phases.

regarding your statement about PSCC being 1.732 x measured value, i used to beleive this also, but i was reading a post on the IET forum by Geoff Blackwell himself, and apparcently, the x2 is because of a 3phase fault, ie a fault across all three phases at once. Its highly unlikely IMO, but never the less, a valid point.

SirKit Breaker · Jul 30, 2012

p11jor said:
Hi all I passed my 2391 about 6months ago and thought I was flying, I know test procedures & how to carry out the tests like the back of my hand and I fully understand any results I get and how to rectify or diagnose any faults in a electrical system.....but then again I have only tested ddomestic installations.

We have recently been doing some wiring on a site, 55 flats & 3 shops below, we're would you even start on this!? (I am not testing it by the way before anyone jumps on me) it's been tested a few weeks back but was so busy on there I couldn't get to put my eyes over what they did.

The apartments had a 10mm swa from the plant to each flat, so I understand Would have to take a ZDB for the swa ze + (r1+r2) of swa
the apartments are a doddle
were would the Zdb reading be noted? Would the supply cable be noted on the schedule of results for the appropriate apartment but marked as a supply cable instead of a final?

Or would it be marked down on the test sheet for the plant room, again what tests would be carried out in the plant room with all the kwh meters, the 125mm armour coming in would they take a Ze from this? How would the pfc work, would you go from all three phases and record highest?

I really don't have the knowledge or experience I thought from the 2391 and this was certainly not covered!

Cheers guys

For a big installation, you would break it up into smaller parts.
Start at the beginning, and then work through your submains from DB1, then test each branch and its final circuits. No different to testing a small domestic set up, its just a lot more involved, and at first glance can look a bit complex. If you do the inspection first like you should do, this will help you to work out a plan of attack for the testing. Take your time with it, and you will be OK.

Cheers..........Howard