For example a meter box with a neutral stud and mcbs per metered supply to their own men switchboard.
Or will this distribution point have to be a main switchboard central supply and all metered supplies be to dbs.
Obviously the second option is better practise
Iv seen many older sites like the first option
I feel like iv read somewhere the first switchboard must be MEN.
And this metering point a switchboard?
1.4.91 says to supply sub circuits and sub main so if its to supply mains then maybe not
I'm not asking what's better practise because obviously a main switchboard central supply supplying metered dbs is best but want to kniw is that option still compliant.
Thanks in advance
As per definition; it can;t be a 'switchboard" unless it supplies - & protects - either submains or final subcircuits.
So if there are no circuit protection devices, or only for mains, then it isn't a switchboard.
If it's not a switchboard, it can't be the MSB.
Which means it can't include a MEN link, and while it may include one or more isolating switches (eg for isolation of meters) they aren't "main switches".
Basically the options under current rules are
a) set the common metering point up as MSB, c/w main switch(es), MEN, MEC, & electrode ,and provide protection for submains out to the various parts (ie a multiple installation) ; or
b) set it up as metering point only, with the (separate) mains for the various installations just passing through (ie a single installation ).
For option a), the outgoing submains will have PECs (unless going to outbuildings).
For option b), there may be no overcurrent protection devices for all or some of the outgoing mains; and the incoming will either be separate mains from the several PoSs somewhere upstream, or possibly in common shared cable / enclosure with the PoSs being in the meter enclosure (in which case they are not "mains" but private works).
Which brings in the other key aspect: the definition of the PoS.
Remembering that metering has absolutely nothing to do with whether any particular collection of fittings is an "installation" or only a "part installation" - and nor does the ICP system used by networks & retailers to denote their various customers.
- Rating: 16.67%
For example one supply to metering point split into 2 seperate icps and each of the 2 meters with own icp has its own main overcurrent protection before meter (orion requirement meter isolation switch).
Or even a larger supply to metering point then that branched into smaller mains and each individually protected?
Because the overcurrent is protecting mains and not sub mains or sub circuit surely its still not a switchboard?
didn't say there can't be overcurrent devices on mains at a metering point; just that there may be none.
Which is absolutely normal where there's no reduction in CCC (eg for a controlled mains for water heating); and overload protection is either at origin (pillar / pole / pit) or at MSB.
Requirements set by retailers / networks / metering providers are one instance; but these are generally extra to requirements in Wiring rules.
And often completely unjustified technically; but enforceable through supply contract with consumer.
But am I correct in saying there is nothing stopping many MEN points in the same structure if they are completely separate installations?
An example it's very common to see 4 or 8 units all in the same structure. All have own seperate supply from network boundary, own MEN MSB and all have there own earth electrode at the front door meters away from each other.
Not just a few; sometimes several per floor over many floors.
Each must have own "main switch(es), MEN, and MEC; but can share electrode
Let's assume it's just setup as a metering point (option b), so it's not a switchboard, but only has the meters, a common neutral link, and fuses/CBs for the multiple outgoing supplies to different MEN installations within the same building.
Where do you get the earth to earth the enclosure of that metering point?
Think big metal box with multiple meters for different installations within the same building/structure. Or similar.
For a single installation earthing the meter box isn't a problem. You just run a PEC back from the earth bar the same way it has been done for decades (but use the correct size earth, not the 2.5mm² earth from the ripple switch wire like it was done years ago).
I'm asking where you should get the earth from when there are multiple installations.
First would need to establish that it is actually a collection of individual installations; and not a multiple installation.
And that means being certain WRT location of the POS(s).
Bearing in mind that neither metering nor ICP system for identifying consumers has any direct bearing on the relevant definitions.
Looking at where the fittings involved change from "non-exclusive fittings" (supply up to that enclosure, the common N-point) to "exclusive fittings" (mains outgoing to each installation); POS for each is most likely within the enclosure.
This enclosure would not be part of any of the installations.
Would be equivalent to a street pillar box; ie part of works.
Therefore would be out of scope for '3000"; which only applies to installations.
Not sure what the applicable rules for works are, and that option may be restricted.
However that's what we'll end up doing if / when TT is introduced; because the installation's earthing system will be separated from the incoming PEN, and the only way to provide fault protection up to & including MSB enclosure, will be by earthing to the PEN of mains.
- Site Admin
- Posts: 162
- Joined: Sun Apr 12, 2020 4:45 pm
- Has thanked: 15 times
- Been thanked: 43 times
What else do you do?
That's because the definition of "electrical installation" that matters is not the one in "3000", but rather the one in the Electricity Act.
That ties "installation" to PoS, and PoS is tied to "exclusive fittings" supplying a "property".
As a result, unless the structure / area has been formally subdivided into more than one property , we can't have a single supply to multiple installations. Unit titles don't count for this purpose; let alone mere tenancy agreements.
Can still have more than one installation on / in a single property, but each must have its own PoS, mains, etc
For any arrangement that splits power among occupancies that are on / in same property, the POS is where the supply enters the property.
There are exceptions for location of PoS, eg distributor's tx within property or specific agreement;
but they don't change the fact that the PoS is set not by choice / interpretation, but by the fact of where the first "exclusive fittings" are.
Everything downstream of a PoS is part of the same installation.
Arrangements that were common under old Acts & Regs; are not necessarily compliant under today's rules.
They can continue in service under ESR 113; but that doesn't make it OK to copy them for a new installation.
The problem with earthing a common meter enclosure (or anything else "common" to a number of installations / part installations), is that there's a risk of losing the earthing due to changes made at the downstream point.
Look at Fig 5.3 and the associated clauses; it's generally not permitted to earth any item to a downstream point.
2018 edition brings in requirements for outbuildings with more than one supply (eg different occupancies) to earth each to the upstream source; and not use PEN submains. Similar for bonding; slabs etc have to either be bonded back to upstream source, or to each of the occupancies.
All so that changes made on one part don't remove the earthing / bonding for other parts.
We bend this general rule by earthing meterboxes, mains support brackets, etc to (downstream) MSB / MEC; and that's OK where there's only one swbd fed from the item requiring earthing.
Clause 220.127.116.11 was amended for 2018 edition so that it applies to "all" PECs;
whereas 2007 edition only covers PECs for submains / final subcircuits, and was silent WRT earthing fittings of mains.
This change clarifies that the only acceptable place to earth fittings related to mains is 'the" MSB. Not "a" MSB, but "the" MSB.
Because, of course, for any supply, there can be only one MSB.
And it also means that "bending" of the general rule, that we've been doing for years, is specifically recognised as acceptable.
But earthing a common meter enclosure to just one of several downstream installations / part-installations is not acceptable.