UK Where to measure Ze, opinions welcome!

[LR1397]

Where do people record the Ze & PFC of a 3 phase installation, if there is say 10m of 25mm tails between the 4 pole isolator (next to the meter) and the 3 phase distribution board, therefore some (though negligible) resistance?

Do you tend to test at the origin, and record that result, or do you regard the MET as being at the DB (as an extension of what is at the meter position) and do your tests there?

Or both, but only use the one at the origin as a reference, and not record it?

Interested to see what people make of this!

 

[Dave OCD]

I'd say ideally it should be recorded right at the origin of the supply in the building as there's a separate part on Schedules of test results for 'zDB'.
But if there isn't a main isolator at the intake it wouldn't be practical with no access to line conductors without removing a DNO fuse...
On an installation where the DNO fuse, meter and CU are right next to each other I'll generally just do the Ze at the CU main switch incoming terminals, with all main bonding conductors disconnected.

 

[westward10]

You take it at the origin and not 10m down the line.

 

[HHamer]

I always do it at the bottom of the main switch of the DB, technically that's where the "installation" starts, as any other DB would require another cert. Record the highest reading achieved out of 3 phases (should be the same or very close to each other, same as the PFC).

 

[davesparks]

Ze is the external earth fault loop impedance and should be measured at the supply terminals, not at any other arbitrary point within the consumers installation.


Also tails should not be 10m long, the 3m rule is applicable to all tails, not just domestic installations!

 

[loz2754]

If it's not convenient to switch off the supply so you can disconnect the earthing conductor in order to measure the Ze at the origin, then you cannot measure the Ze at all. You can determine the declared Ze from the DNO, and you can measure the Zs at the DB.

 

[HHamer]

If it's not convenient to switch off the supply so you can disconnect the earthing conductor in order to measure the Ze at the origin, then you cannot measure the Ze at all. You can determine the declared Ze from the DNO, and you can measure the Zs at the DB.

Quite a common practice, especially in the office blocks where one of the breakers i marked up "server room"

 

[nicebutdim]

Quite a common practice, especially in the office blocks where one of the breakers i marked up "server room"

Probably even more common in industrial settings, where isolating one breaker could result in seven figure losses, but that doesn't make it right.