Interesting talk with DNO engineer

[davesparks]

Not the outgoing side of the meter. That’s usually sealed.
Outgoing side of isolator if they have one, or supply side of mainswitch, or an accessible Henley if there’s no seals.

basically, if you need to cut a seal to access a terminal, you don’t do it.
likewise cutting a seal and pulling a fuse just to find out the size. If it’s not printed on a sticker, then it’s “not verified”

I'm used to seeing a lot of self connect meters, the outgoing terminals are not sealed and the meter has a built in isolator.

 

[Chivers]

There's normally an Isolator at the top of our Local equipment and then a Block Terminal on a Din rail. So I never cut the seal to measure the ZE, just wanted others opinions as my colleague was insistent we should cut the seal and remove the fuse! I measure it from the block terminal as the resistance of the 25mm copper upstream is negligible especially as it's barely two feet long. If the measurement is too high then Isolator top then over to the DNO...

 

[Simon47]

IMO you are right, and anyone suggesting that they "must" cut the seals to measure at the DNO terminals is just plain wrong (as well as acting both illegally and dangerously).
Firstly, as you point out, a couple of feet of 25mm2 cable will have less effect on readings than teh variability of the contact resistance of your measuring leads. Unless you were to employ precisions 4-contact measuring gear, then that few feet of cable reaslly is insignificant.
Secondly, we are normally measuring Ze (and Zs) so that we can calculate a maximum value for R1+R2 and R1+Rn in order to ensure correct operation of protective devices. If you are so paranoid as to want to account for those 25mm2 tails, then you need to include them in the calculation - and measuring Z at your main switch is the easiest way of doing that. So really, an insistence of measuring it at the DNO terminals is wrong in the technical sense as it's excluding part of the circuit from the measurements and calculations. Being pedantic, you could measure Z at the DNO terminals, then calculate the resistance of a few feet of tail, and add the 2 together to get Zdb