Redundant Isolation Switch in combination boards (meter + MSB)

This includes types of overcurrent, Short Circuit and RCD protection
Post Reply
Posts: 200
Joined: Tue Apr 14, 2020 11:08 am
Has thanked: 33 times
Been thanked: 10 times

Redundant Isolation Switch in combination boards (meter + MSB)

Post by JamieP »

Under Orion's network for Christchurch, you are required to provide overload protection for mains and also a isolation device before the meter

Usually this is in the form of a 63A MCB in the meter enclosure before the meter and then the meter feeds off to the MSB and MS (din mount isolation switch)

I've seen this set up replicated in combined boards with a 63A MCB isolation switch before the meter and then a main switch after feeding everything else, I personally feel the 2 devices aren't needed and you could simply make your 63A MCB before the meter your main switch and omit the other device, I think it's done this way just out of habit of them being separate usually and people not actually thinking about the change in situation

As far as I'm aware there's nothing that says a meter cant be after the main switch but more so that they are allowed to be before

This single device (63A MCB) ticks all the boxes to me, it gives a main switch, overload protection for the mains and a isolation device for the meter but I'm just wondering if anyone can fault me here?
Posts: 18
Joined: Tue Apr 14, 2020 9:22 am
Has thanked: 0
Been thanked: 4 times

Re: Redundant Isolation Switch in combination boards (meter + MSB)

Post by pluto »

What ie required is NOT detailed in as/nzs 3000 it is the agreement between the metering provider and the electricity retailer. as/nzs 3000 excludes revenue metering installation.

I agree that main switch when operated should remove supply from nearly all of the electrical installation. Most approved revenue metering configurations are designed by the revenue metering providers to suit them and not tthe consumer.

The way most metering providers have the main switch fitted, still leaves about 40% of the electrical installation alive when the main switch is off.
Posts: 335
Joined: Thu Apr 16, 2020 11:24 am
Answers: 1
Has thanked: 1 time
Been thanked: 107 times

Re: Redundant Isolation Switch in combination boards (meter + MSB)

Post by AlecK »

The requirement for o/l protection may be spelt out in Orion's (and some other) network connection contracts; but it's always our responsibility because mains are part o9f the installation; and all conductors in the installation must have o/l protection, short circuit protection, & fault protection. ESRs require networks to provide s/c & fault protection for mains (since those can only be provided upstream) but they're not required to provide mains with o/l protection .
That said; many networks provide de-facto o/l protection, by fusing to prevent the installation from drawing more than the contract capacity.
We can take advantage, and size the mains using contract capacity as the load-to-be-carried; whereas the actual load-to-be-carried is often much less.

Revenue metering is not excluded from "3000". It's just that the Standard sets no specific requirements for it. However it's part of the installation (as defined in Electricity Act) and therefore the same general requirements apply.
That said, allows it to be not controlled by the main switch(es).

The contractual (as against Regulatory) requirement for ability to isolate the metering is because many people who may be required to work on metering won't have authority to touch the network's supply fuses in order to work safely. It avoids them having to call in the network for safety isolation / re-livening.

Correct that where the metering is located at MSB, and it's a simple installation, a single mcb can perform all 3 functions - at least for the incoming mains. Care needed to ensure all rules for "main switch" are complied with. By contract, a fuse can only provide 2 of the 3 functions.
Not so where safety services require additional "main switches"; or where metering isn't at MSB.
Also note that there are often smaller cables from metering to MSB for controlled loads (eg storage heating, water heating). These are also "mains" (common neutral) and the "single" mcb generally won't protect them from o/l. However often the load(s) & configuration are such that o/l isn't possible.
Post Reply