Earthing/bonding Ceiling Lamps And Ceiling Support Structures.

[petercsea]

Ceiling height 276cms. Flush fitting metal framed lamp holders not in contact with the aluminium frame holding the ceiling boards. The ceiling is the style of dry board suspended in aluminium trays. The aluminium structure of the ceiling is supported by wires from the metal trusses of the roof structure.

The wiring is free in that it lays across the ceiling to the light fittings. Your thoughts on earthing or not?

Thanks once again.

[Crossy]

The latest UK regulations (17th Edition + Part P) say that all metallic light fittings must be earthed, regardless of height (unless double insulated). But, we're not in the UK and the lights are (just) out of reach so the risk of shock is very small.

Personally, I'd earth them. For the small extra cost of the cable it also gives the presence of an earth should you later add a fan (which will almost certainly need a ground).

As to grounding the roof structure / ceiling supports there is little mention in the UK regs. I would ground your roof structure to a seperate ground rod placed away from your mains ground. The idea being that if you get a lightning near miss you won't be feeding nasties into your electrical system, but if you do get a fault that could cause your ceiling stucture to become live there will be enough ground current to take out the breaker.

[petercsea]

The latest UK regulations (17th Edition + Part P) say that all metallic light fittings must be earthed, regardless of height (unless double insulated). But, we're not in the UK and the lights are (just) out of reach so the risk of shock is very small.

Personally, I'd earth them. For the small extra cost of the cable it also gives the presence of an earth should you later add a fan (which will almost certainly need a ground).

As to grounding the roof structure / ceiling supports there is little mention in the UK regs. I would ground your roof structure to a seperate ground rod placed away from your mains ground. The idea being that if you get a lightning near miss you won't be feeding nasties into your electrical system, but if you do get a fault that could cause your ceiling stucture to become live there will be enough ground current to take out the breaker.

Thanks Crossy,

I will go ahead and earth the lights.

I understand what you're sayng about the roof structure and remote earth rod.

Is there any need to consider that all the cables are running over the edges of the aluminium trays. These are in effect 60 cm squares and their edges are quite sharp. When you drag a cable across them as in the initial installation, the physical resistance is very noticeable and leaves a bit of a mark on the PVC insulation. The ceiling space gets very hot and I have measured it at 60C. (I've derated the cables in an attempt to accomodate this fact plus I have left them free and unbundled but the PVC also noticeably softens at this heat. Do you think that there is a significant chance of cable damage leading to a fault because of the "sharp edges" plus the temperature. This is something that concerns me a little bit. The installing electrician seemed very surprised by the question and said not to worry.

Once again thanks.

[Crossy]

Is there any need to consider that all the cables are running over the edges of the aluminium trays. These are in effect 60 cm squares and their edges are quite sharp. When you drag a cable across them as in the initial installation, the physical resistance is very noticeable and leaves a bit of a mark on the PVC insulation. The ceiling space gets very hot and I have measured it at 60C. (I've derated the cables in an attempt to accomodate this fact plus I have left them free and unbundled but the PVC also noticeably softens at this heat. Do you think that there is a significant chance of cable damage leading to a fault because of the "sharp edges" plus the temperature. This is something that concerns me a little bit. The installing electrician seemed very surprised by the question and said not to worry.

Once again thanks.

If you are using TPS cable (that's 2 or 3 cores with an outer overall jacket) then MINOR abrasions to the outer should not be a worry, however if those edges are really sharp enough to cut into the cable I'd be looking to add some form of protection to reduce the possibility.

If you're using individual wires (sometimes called building wire) it should not be left naked as it's (mostly) only single insulated and is intended to be run in conduit, running this type of wire across your sharp edges is potentially dangerous as that insulation has relatively little mechanical strength and low resistance to nicking.

The cable should be marked with a temperature rating (105C is common), this is the maximum temperature that the insulation should be run at (ambient plus any heating due to load). To be honest, unless you have a LOT of lighting and are pushing the current ratings (which you suggest you're not) I doubt you'll run into trouble with lighting cables. 60C is pretty warm for a ceiling space though, maybe some roof insulation and attic ventilation is an idea.

NOTE although not strictly to code (whatever that means in Thailand) I see no reason why you should not run 2 core TPS (easily obtained) and a single ground wire, after all, a nick in the ground isn't normally going to cause an issue (it's certainly less of an issue than having no ground).

[longball53098]

If this lighting wire is the TPS type and just running across the ceiling grid,,,,,,,,,,,,,if there is enough slack to the cables you may try to raise them up several inches and secure the cables to the ceiling grid support wires with plastic cable ties,,,,,,,,maybe every other ceiling tile,,,,,,,,,,,if you plan on adding a earth wire to the lighting then it could be attached at the same time. This method would get the cable off the grid and away from the sharp edges,,,,,,,,,,,,,also helps when someone pops a tile up for looking around when doing any service work,,,,,,,,,,I would not be tightening the ties too much though,,,,,,,,,just enough to hold the cables up.

[Crossy]

If this lighting wire is the TPS type and just running across the ceiling grid,,,,,,,,,,,,,

,,,,,,,,,,just enough to hold the cables up.

Sounds like a smart idea to me. Now why didn't I think of that (makes note in little black book)

EDIT Love the sig. Longball, see examples every single day

Edited May 7, 2008 by Crossy

[longball53098]

Ok thanks,,,,,,,,every now and then the brain kicks in and something good pops out,,,,,,,,,,,

[petercsea]

If this lighting wire is the TPS type and just running across the ceiling grid,,,,,,,,,,,,,if there is enough slack to the cables you may try to raise them up several inches and secure the cables to the ceiling grid support wires with plastic cable ties,,,,,,,,maybe every other ceiling tile,,,,,,,,,,,if you plan on adding a earth wire to the lighting then it could be attached at the same time. This method would get the cable off the grid and away from the sharp edges,,,,,,,,,,,,,also helps when someone pops a tile up for looking around when doing any service work,,,,,,,,,,I would not be tightening the ties too much though,,,,,,,,,just enough to hold the cables up.

Thank you Longball. Yet again the simple answer is a fantastic solution. Your brain works well