Interesting talk with DNO engineer

[Cbuk2k2]

Constant pressure springs - is that the term?

I’d say constant force spring

 

[DPG]

I’d say constant force spring

Close enough then.

 

[Midwest]

I work as a Maintenance Electrician on Highways (not street lighting). I won't go into great detail but we have cabinets with DNO fuses and a meter then normally a two pole isolator. I've been doing this role for almost two years, We were transferred / Tuped across to another firm who tendered for the contract. One of the guys who has far more Network testing experience than me (though doesn't have 2391), was insisting a number of months ago that we needed to pull the DNO fuse (cutting seals) to get a true ZE and determine the Fuse rating. Up until then I'd been Testing ZE at the closest point to the meter with Earth removed to test towards the supply and noting the Fuse rating of the carrier (most are sealed anyway), generally the Earth is 16mm or 25mm and is two feet long so of negligible resistance. He still keeps insisting we remove fuses despite being told by hierarchy we should Not do this.

Who's right in this instance? We do have G39 BTW

Doesn't look as if your question been answered.

When testing for a Ze on a domestic property, I would take my reading from the incoming supply to the main switch of a CU. Unless I was replacing the tails, or had to isolate the supply using the main fuse, I would not cut the seal. Therefore I would not be able to verify the size of the main fuse. I would document what was written on the fuse carrier, noting I had not inspected the fuse itself.

Is there a particular reason, you need to identify the size & rating of the main fuse, in your case?

In my case my DNO allows Scheme members to remove their main fuse (with appropriate PPE), and reapply a temporary seal, provided by them and furnishing them with the details. Not all DNO's do though.

 

[davesparks]

I work as a Maintenance Electrician on Highways (not street lighting). I won't go into great detail but we have cabinets with DNO fuses and a meter then normally a two pole isolator. I've been doing this role for almost two years, We were transferred / Tuped across to another firm who tendered for the contract. One of the guys who has far more Network testing experience than me (though doesn't have 2391), was insisting a number of months ago that we needed to pull the DNO fuse (cutting seals) to get a true ZE and determine the Fuse rating. Up until then I'd been Testing ZE at the closest point to the meter with Earth removed to test towards the supply and noting the Fuse rating of the carrier (most are sealed anyway), generally the Earth is 16mm or 25mm and is two feet long so of negligible resistance. He still keeps insisting we remove fuses despite being told by hierarchy we should Not do this.

Who's right in this instance? We do have G39 BTW

Ze is taken at the consumers supply terminals, this is the point at which the consumers tails connect to the suppliers equipment. This will either be the outgoing side of the meter or the isolator.

 

[littlespark]

Ze is taken at the consumers supply terminals, this is the point at which the consumers tails connect to the suppliers equipment. This will either be the outgoing side of the meter or the isolator.

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”

 

[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