ZE Test Question

[davidjtown]

Newbie testing question:

Two consumer units in same location.
Tails approximately 2 metres.
MET bar next to meter in another room.
Bonding attached to main consumer unit.

Where should I remove the earth from, consumer unit or MET? If MET, use a wander lead? My thoughts are to disconnect earth from MET due to parallel paths but not as practical.

 

[Baddegg]

You need to eliminate the parallel paths mate to test the Ze, ie external impedance....so disconnected main earth from met...what system are you testing?

 

[davidjtown]

You need to eliminate the parallel paths mate to test the Ze, ie external impedance....so disconnected main earth from met...what system are you testing?

TNCS. So you’d obviously use a wander lead to test against the live on the Consumer unit?

 

[buzzlightyear]

I agree above, and don't forget to isolate first.

 

[Baddegg]

Or disconnect all the other parallel earths at the MET and test at the unit...assuming the main earth comes from the service head to the met and on to the Cu obviously....

 

[westward10]

I would disconnect the earthing conductor at the CU, 2m is negligible.

 

[buzzlightyear]

Or disconnect all the other parallel earths at the MET and test at the unit..

It depends the old met bar, bolts.
Quicker to drop out of the cu.

 

[pc1966]

Trying to remove parallel paths from downstream of the CU can be hard: easy to disconnect main bonding for gas & water as they are fairly obvious but probably they are still bonded via CPC of boiler supply, immersion heater, ignition of gas cooker, etc.

So as above, isolate the loads and remove the supply earth from the CU to measure Ze to it.

 

[davidjtown]

I will be testing the secondary CU. It only has the earth from the MET attached to it (no bonding). In the MET is just the supply earth and one earth to each CU. Bonding is connected to the primary CU (which isn’t being tested). I assume it’s therefore ok to isolate both CUs, disconnect the primary CU from MET, disconnect earth from secondary CU and test at the CU? This eliminates the need for a wander lead. All parallel paths therefor eliminated. Is that acceptable?

 

[westward10]

Disconnect at both CUs and test to the conductor at the CU.

 

[Rockingit]

If it's a PME head and only 2m's, surely this would be quickest by isolating the loads and just dissing the earth as it comes out of the head/MET??

 

[davidjtown]

Thanks for the replies. Even though it’s only 2 metres, it is quite a distance to run a lead to the MET as it’s the other side of the wall.

 

[Strima]

Isolate, drop the main earthing conductor out of the MET and test from there using the tails form one of the boards for the line supply.