Power point in external glasshouse

cliffed · Apr 4, 2022

Client wants 2g socket in greenhouse, obviously IP rated,will be supplied from Rcd, the greenhouse consists of metal frame work on the ground.
Does this require MPBC

cliffed · Apr 8, 2022

Nice discussion agree & disagree on so many issues…typifies the interpretation of each of us on actually how to be compliant.
This all for a 2g power point, from an existing external radial circuit 2.5mm.

cliffed · Apr 8, 2022

Julie. said:
But both 544.1.1 and 54.8 have the caveat "except where PME conditions apply"

That is the crux of the matter.

I take it to mean if the conductors are part of the PME system (i.e. the combined part) then table 54.8 and 10mm^2 or 6mm^2 etc do not apply, and no figures are specified within the wiring regs.

In all other cases - where the conductors have been separated (after the service head) then the conditions in table 54.8 and 544.1.1 are applicable.

If it did just simply mean "except for PME" then that would be the wording.

Why write the clause as particular to PME conditions, if they didn't mean where it is part of the PME (TN-C ) aspect.

Taking "except where PME conditions apply" to mean you can't use that table or regulation when dealing with part of the PME system fits the words, and the fact that the bonding size isn't specified.

Taking "except where PME conditions apply" to mean that the bonding size in the regulation and table isn't applicable to an installation supplied by a PME system begs the questions 1) what cable size should be used as 54.8 is "except where PME conditions apply" so what size for PME; and 2) why the "odd" wording, why not just say 'except for installations supplied from PME"?

Except..
where the cross sectional area required not less than half…..
Except..
where highway furniture…..
Where PME conditions the MPBC in accordance with the PEN Table 54.8
Where an installation has more than…..MPBC Sselected to the largest PEN of the supply
Must say a 2 year old could write this better & most probably understand it better too.

mainline · Apr 8, 2022

cliffed said:
Except..
where the cross sectional area required not less than half…..
Except..
where highway furniture…..
Where PME conditions the MPBC in accordance with the PEN Table 54.8
Where an installation has more than…..MPBC Sselected to the largest PEN of the supply
Must say a 2 year old could write this better & most probably understand it better too.

It looks to me like nothing has changed apart from the wording.

How far away is the greenhouse ?

Explain to the customer how much easier it would be to use an extension lead

cliffed · Apr 8, 2022

mainline said:
It looks to me like nothing has changed apart from the wording.

How far away is the greenhouse ?

Explain to the customer how much easier it would be to use an extension lead

Beauty… extension leads with a IP …SPD…Rcd…AFDD & free bogof bonding clamp

mainline · Apr 8, 2022

cliffed said:
Beauty… extension leads with a IP …SPD…Rcd…AFDD & free bogof bonding clamp

Probably some Chinese kit already on the shelf at Aldi

Julie. · Apr 8, 2022

Yes, my apologies, I had read that both aspects were except PME, but it in fact is the same as previously, 54.8 applies to PME, and the first part of 544.1.1 to non-PME; it is only that portion which has changed to allow smaller bonding size for additional buildings.

My fault entirely.

cliffed · Apr 8, 2022

Julie. said:
Yes, my apologies, I had read that both aspects were except PME, but it in fact is the same as previously, 54.8 applies to PME, and the first part of 544.1.1 to non-PME; it is only that portion which has changed to allow smaller bonding size for additional buildings.

My fault entirely.

Great confusion eliminated

westward10 · Apr 8, 2022

Took a fair bit of deliberation on my part before I finally said I don't see it that way Julie, I then hid behind the sofa

Julie. · Apr 8, 2022

westward10 said:
Took a fair bit of deliberation on my part before I finally said I don't see it that way Julie, I then hid behind the sofa

If I am wrong, I am wrong, no issues with that.

I looked right through the changes when I got the new version, but obviously didn't digest it all properly.

UNG · Apr 8, 2022

After 84 posts can he fit the socket yet

mainline · Apr 9, 2022

UNG said:
After 84 posts can he fit the socket yet

Customer cancelled, they said he spent too much time on his phone

cliffed · Apr 9, 2022

UNG said:
After 84 posts can he fit the socket yet

Think I’ll give it a miss

LastManOnline · Apr 10, 2022

cliffed said:
Does this require MPBC

No.

Why? MPBC assumes the extraneous part can.....

1) introduce a potential (yes)
2) introduce heat damaging currents. (No)
3) can be simultaneousely touched with other extr. metal (No)

Does it require supplementary bonding? Yes

Why? A class 1 appliance may be plugged in to the powerpoint

cliffed · Apr 10, 2022

LastManOnline said:
No.

Why? MPBC assumes the extraneous part can.....

1) introduce a potential (yes)
2) introduce heat damaging currents. (No)
3) can be simultaneousely touched with other extr. metal (No)

Does it require supplementary bonding? Yes

Why? A class 1 appliance may be plugged in to the powerpoint

It an extraneous & needs bonding with the appropriate sized conductor

[ElectriciansForums.net] Power point in external glasshouse

cliffed · Apr 10, 2022

LastManOnline said:
No.

Why? MPBC assumes the extraneous part can.....

1) introduce a potential (yes)
2) introduce heat damaging currents. (No)
3) can be simultaneousely touched with other extr. metal (No)

Does it require supplementary bonding? Yes

Why? A class 1 appliance may be plugged in to the powerpoint

It an extraneous & needs bonding with the appropriate sized conductor

LastManOnline · Apr 10, 2022

cliffed said:
It an extraneous & needs bonding with the appropriate sized conductor

For the illustration you posted supplementary bonding would suffice. The drawings replicates how a kitchen sink was traditionally bonded to the socket where the electric kettle was plugged in

mainline · Apr 10, 2022

LastManOnline said:
For the illustration you posted supplementary bonding would suffice. The drawings replicates how a kitchen sink was traditionally bonded to the socket where the electric kettle was plugged in

Just wondering how you would connect the bonding

LastManOnline · Apr 10, 2022

mainline said:
Just wondering how you would connect the bonding

As the OP describes the installation, there are no extraneous parts except for the metal frame. Makes bonding very simple. The metal frame will provide a path to earth but it will likely be relatively high resistance (should be tested and established). A bonding conductor of 4mm from the socket to the metal frame will be more than adequate.

mainline · Apr 10, 2022

LastManOnline said:
As the OP describes the installation, there are no extraneous parts except for the metal frame. Makes bonding very simple. The metal frame will provide a path to earth but it will likely be relatively high resistance (should be tested and established). A bonding conductor of 4mm from the socket to the metal frame will be more than adequate.

Maybe it's not a good idea to export the PME to the metal frame ?

pc1966 · Apr 10, 2022

It is really about the relative risks of each of the options:

TT avoids the whole open-PEN fault issues of high touch potential and possibly high sustained currents in the CPC irrespective of RCD/MCB protection. But in turn you are dependent on the RCD electronics and light-current trip mechanism to disconnect on any fault.
TN gives you the reliable high fault current option to disconnect on the OCPD side of things, but in turn you have the issue of differences in CPC/Earth potential leading to the issue of bonding of extraneous parts, etc, open-PEN magnitudes of currents, etc.

In this situation I would probably go with TT due to the 2*1.5mm CPC of the supplying RFC and what that could mean for open-PEN faults. Though I would be tempted to put a RCD style socket as TT on the existing socket's supply so no single RCD failure for anyone plugging stuff in.

mainline · Apr 10, 2022

pc1966 said:
It is really about the relative risks of each of the options:

TT avoids the whole open-PEN fault issues of high touch potential and possibly high sustained currents in the CPC irrespective of RCD/MCB protection. But in turn you are dependent on the electronics and light-current trip mechanism to disconnect on any fault.

TN gives you the reliable high fault current option to disconnect on the OCPD side of things, but in turn you have the issue of differences in CPC/Earth potential leading to the issue of bonding of extraneous parts, etc, open-PEN magintude of currents, etc.

In this situation I would probablygo with TT due to the 2*1.5mm CPC of the suppling RFC and what that could mean for open-PEN faults. Though I would be tempted to put a RCD style socket as TT on the existing socket's supply so no single RCD failure for anyone plugging stuff in.

Agreed, TT is the way to go.

pc1966 · Apr 10, 2022

LastManOnline said:
As the OP describes the installation, there are no extraneous parts except for the metal frame. Makes bonding very simple. The metal frame will provide a path to earth but it will likely be relatively high resistance (should be tested and established). A bonding conductor of 4mm from the socket to the metal frame will be more than adequate.

That is one aspect that never seems to be covered as "extraneous conductive" could range from sub-ohm with seriously high open-PEN currents being possible (e.g. metallic service pipes shared with other DNO circuits) to the kOhm range when any mechanically sound bond wire will do.

I guess the old 10mm, etc, rule is about assuming the worst (but still realistic) case.

LastManOnline · Apr 10, 2022

pc1966 said:
I guess the old 10mm, etc, rule is about assuming the worst (but still realistic) case.

Completely agree. In the domestic sector "bonding" and "10mm" have become synomimous. Interchangeable. Causing unfortunately a lot of confusion and unnecessary expense in the process.
Take this particular post for example. Its revolved around whether the OP should run a 6mm or a 10mm main bond?. In reality, neither a 6 or 10 mm is required.
Unfortunately the regulators have inadvertently created a situation where the confusing issue of bonding, which causes so many headaches, looks set to continue causing headaches for the next generation of sparks

cliffed · Apr 11, 2022

mainline said:
Agreed, TT is the way to go.

Agree TT is the way, it’s been a epic post with many points of view.
This just proves that adding a power point to the greenhouse needs a lot of thinking & cost wise, gonna be expensive.
The client literally told us how to connect the power, from an existing external socket.
Shes in for a surprise when priced for TT system, Rcd DB, etc

pc1966 · Apr 11, 2022

cliffed said:
Agree TT is the way, it’s been a epic post with many points of view.
This just proves that adding a power point to the greenhouse needs a lot of thinking & cost wise, gonna be expensive.
The client literally told us how to connect the power, from an existing external socket.
Shes in for a surprise when priced for TT system, Rcd DB, etc

So are the existing sockets not on RCD at all?

If they are on DP RCD/RCBO then all it needs is the local earth rod (and I presume bonding that to the greenhouse frame).

I don't have amendment 2 book yet, but have they allowed the RCD socket outlets to be used again for "additional protection" and/or what other restrictions are still in place for that?