Steel Container

[ChrisElectrical88]

A client has recently had a 10ft x 6ft Steel Cabin delivered. It will be a permanent addition for a block of flats outside for storing things in for contractors. They want a light inside and a floodlight outside.
Currently next to it about 5 yards away is a bin store which has a light in it from the main building which is on a 6A supply. The earthing System is PME. I can’t run a dedicated supply from the main building due to the amount of paths and obstacles now in the way.
Not keen to continue the PME into the steel container as it’s permanent, I’d rather TT. I have thought about coming from the Bin store supply into a 3A FCU, TTing the outgoing supply here and going into the cabin with this. Any thoughts?

 

[Strima]

I'd use an RCD FCU.

 

[ChrisElectrical88]

I'd use an RCD FCU.

Yes it would be a RCD FCU. Sorry should have put that.

 

[ChrisElectrical88]

Completing the job tomorrow. Nice and easy.
just a quick one, the steel container isn’t extraneous, I’ve only used class 2 light fittings inside. Does the steel container need a fly lead from the fused spur or would you just leave it?

 

[pc1966]

I think a steel box in contact with the Earth very much is extraneous as far as earthing is concerned!

So yes, if it were me the steel box should be bonded to the earth system inside the container.

 

[ChrisElectrical88]

Of course it is extraneous ?. I’ll take a bond from the earths at the spur to the steel itself then .

 

[Ian1981]

I’m not sure that RCD spurs are compliant as they are not included in bs7671 and bs 7288 state that they only provide supplementary protection and that additional protection is assumed upstream.

 

[suffolkspark]

Of course it is extraneous ?. I’ll take a bond from the earths at the spur to the steel itself then ☺.

You'll need a correctly sized bond back to the MET if it is

 

[Ian1981]

How can a steel box forming the enclosure of the spur be an extraneous conductive part by definition,when it forms part of the electrical installation? Is it not an exposed conductive part rather than an extraneous conductive part ?

 

[ChrisElectrical88]

You'll need a correctly sized bond back to the MET if it is

There will be. The MET will be a 6mm Earth in the fused spur from the Rod. I’ll tap a 6mm earth from there onto the container.

 

[telectrix]

Of course it is extraneous ?. I’ll take a bond from the earths at the spur to the steel itself then ☺.

How can a steel box forming the enclosure of the spur be an extraneous conductive part by definition,when it forms part of the electrical installation? Is it not an exposed conductive part rather than an extraneous conductive part ?

wrong... by that you would have to say that any room with electrical equipment fixed to the walls, is a part of the installation.

 

[Ian1981]

wrong... by that you would have to say that any room with electrical equipment fixed to the walls, is a part of the installation.

So you are saying that a metal enclosure on a metal clad socket is not an exposed conductive part??
Why would a wall be part of the electrical installation?

 

[telectrix]

yes. it's extraneous because it is not a part of the electrical installation.

 

[Ian1981]

yes. it's extraneous because it is not a part of the electrical installation.

Of course it’s part of the electrical installation, it forms the enclosure of the spur and is exposed ie class 1 an exposed conductive part.

 

[telectrix]

Of course it’s part of the electrical installation, it forms the enclosure of the spur and is exposed ie class 1 an exposed conductive part.

afraid we have to disagree on this one. anyway Zebedee says "time for bed",

 

[Ian1981]

afraid we have to disagree on this one. anyway Zebedee says "time for bed",

Good night Tel x

 

[ChrisElectrical88]

Of course it’s part of the electrical installation, it forms the enclosure of the spur and is exposed ie class 1 an exposed conductive part.

The spur is acually going on the bin store wall for accessibility issues. Only thing inside the cabin is a class 2 microwave fitting and a class 2 floodlight.
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I installed the spur and rod a while ago. The SWA put of the side is the outgoing waiting to serve the cabin.

 

[pc1966]

How can a steel box forming the enclosure of the spur be an extraneous conductive part by definition,when it forms part of the electrical installation? Is it not an exposed conductive part rather than an extraneous conductive part ?

If it is:

• Conductive (clearly it is!)
• Liable to introduce a potential (as it is in contact with the true Earth, it can do so)
• Not part of the electrical system (which without any bonding it is)

It would be 'extraneous'.

The key point is the contact to the outside world (i.e. true Earth). Now in a TT setup one would like to imagine that the earth rod and the box's contact are both at the same potential, but an external fault could introduce a worrying touch potential between the two.
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I installed the spur and rod a while ago. The SWA put of the side is the outgoing waiting to serve the cabin.

Nice to see the detailed label!

 

[DPG]

I don't see how the cabin forms the enclosure of the FCU.

 

[Ian1981]

I can’t see why the cabin requires bonding at all.

 

[pc1966]

I can’t see why the cabin requires bonding at all.

Because it is no different from a metal service pipe.

OK, the size and shape are not quite the same, but it is a metal thing in contact with the ground outside. Just ask any topologist (after they stop wondering if the doughnut and tea cup are any different).

 

[Ian1981]

Because it is no different from a metal service pipe.

OK, the size and shape are not quite the same, but it is a metal thing in contact with the ground outside. Just ask any topologist (after they stop wondering if the doughnut and tea cup are any different).

If it had it’s foundations buried in good contact with true earth , then yes id agree.
A metallic service pipe is buried directly into the ground and enters an electrical installation.
There is also no accessible exposed conductive parts in the cabin.

 

[pc1966]

If it had it’s foundations buried in good contact with true earth , then yes id agree.

I think that if, on a wet day, it is <= 20k ohm to earth then it should be considered in contact (extraneous) as then you could get a >= 10mA shock. Now 10mA alone is not likely fatal, but it is going to lead to invoulatery action that could cause injury.

There is also no accessible exposed conductive parts in the cabin.

Perhaps for now. But given the small cost & effort to bond them and make absolutely sure I would just do that.

 

[bigspark17]

How is the cabin any diferent to a metal hand rail on a pateo? If you put a light on that would you TT the railing? How about a picture of the cabin sitting on ground and. Test from MET to the cabin.

 

[ChrisElectrical88]

Well today is a very wet day and I’m doing the cabin today so I will IR test between the metal work and container and let you know the results. I will bond it anyway as it’s that easy to do and knowing the numptys round here someone will cock something up adding something in.

 

[ChrisElectrical88]

Cabin went closer than expected. Tested at 26kOhms in the damp so probably extraneous in heavy rain ?. Bonded it anyway, only took 10 minutes
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Probably get slammed here for not putting the bond in a conduit box ?

 

[pirate]

I followed this video, and installed a separate 10mm cpc from the metal shed that was having power installed. The SWA incomer goes to an RCD fcu then to a couple of sockets, as there is no RCD at the CU. The 10mm goes back to the main earth terminal beside the board. The SWA is 3 core with one core as cpc, and the armour is bonded both ends because that was easy to do.

View: https://www.youtube.com/watch?v=NvENsVUhDUs

 

[ChrisElectrical88]

I followed this video, and installed a separate 10mm cpc from the metal shed that was having power installed. The SWA incomer goes to an RCD fcu then to a couple of sockets, as there is no RCD at the CU. The 10mm goes back to the main earth terminal beside the board. The SWA is 3 core with one core as cpc, and the armour is bonded both ends because that was easy to do.

View: https://www.youtube.com/watch?v=NvENsVUhDUs

Interesting. I however wouldn’t have a chance to get a cable back to the MET, hence why I tapped off the bin store light.
In an ideal world I would have brought over a dedicated circuit with a small CU inside the cabin itself.

 

[pirate]

I should add that the installation to the metal shed was easy enough for the SWA, but the 10mm had to take a toruous route through the basement of the house, which took quite a long time to complete. We ran a cat5 in at the same time...welll, you never know when you might need one!
I must also add that I received considerable guidance on the whole job from an esteemed member of this forum, for which I am most grateful.
I'm not sure the cpc of a lighting circuit would be enough?

 

[ChrisElectrical88]

I should add that the installation to the metal shed was easy enough for the SWA, but the 10mm had to take a toruous route through the basement of the house, which took quite a long time to complete. We ran a cat5 in at the same time...welll, you never know when you might need one!
I must also add that I received considerable guidance on the whole job from an esteemed member of this forum, for which I am most grateful.
I'm not sure the cpc of a lighting circuit would be enough?

There is a Rod in the ground for mine mate and a TT. So 6mm to the spur from the rod and a 10mm from the rod to the container Bonding point. If the lads on here didn’t mention it I probably would have just supplementary bonded it with a 2.5mm off the whisky box where the light flexes are terminated. That probably still would have been ok with a reading of 25kOhms but not worth it when there is only a few kOhns to play with.

 

[pirate]

DOH! I had completely forgotten about the rod...