installing cables through joist where 50mm not possible

[amateurSparky]

The problem I have is two pipes are clipped to the joist thereby not allowing me to drill centre of the joist and complying with the 50mm top and bottom requirement. It runs in parallel along the whole length of the joist. My understanding of the regs is that you could use steel conduit as long as it's earthed. Is this the best/eaiest/quickiest way of doing this? How would I earth the conduit? It's a very short run inbetween two joists

 

[JohnW2]

Where is this joist - below a suspended ground-floor floor, between a ceiling and floor above, or what?

Kind Regards, John

 

[EFLImpudence]

If in an acceptable position regarding the span the top of the joist could be notched and covered with a 3mm. thick steel plate.
N.B. it is not allowed to notch the bottom of joists.

 

[JohnW2]

If in an acceptable position regarding the span the top of the joist could be notched and covered with a 3mm. thick steel plate.

Where does this 3mm figure come from? - 'Safeplates' are only about 1mm thick. 522.6.100(v) merely talks of 'sufficient' protection against nails and screws.

Kind Regards, John

 

[big-all]

The problem I have is two pipes are clipped to the joist thereby not allowing me to drill centre of the joist and complying with the 50mm top and bottom requirement. It runs in parallel along the whole length of the joist. My understanding of the regs is that you could use steel conduit as long as it's earthed. Is this the best/eaiest/quickiest way of doing this? How would I earth the conduit? It's a very short run inbetween two joists

50mm is an electrical safe zone requirement
the structural requirements are different
notching is 1/8th off the top in the mechanical safe zone
holes central are 1/4/25% in the mechanical safe zone

 

[GaryMo]

Where does this 3mm figure come from? - 'Safeplates' are only about 1mm thick. 522.6.100(v) merely talks of 'sufficient' protection against nails and screws.

I'm not sure where it came from originally but it's repeated HERE by the ESC in Q1.12

Q1.12

What types of mechanical protection provide sufficient protection against penetration by nails, screws and the like?

As an example, steel of 3 mm minimum thickness is generally considered to provide sufficient mechanical protection, except where shot-fired nails are likely to be used.

Regulation number(s)

522.6.101
522.6.103

 

[JohnW2]

Where does this 3mm figure come from? - 'Safeplates' are only about 1mm thick. 522.6.100(v) merely talks of 'sufficient' protection against nails and screws.

I'm not sure where it came from originally but it's repeated HERE by the ESC in Q1.12. Q1.12... What types of mechanical protection provide sufficient protection against penetration by nails, screws and the like? ... As an example, steel of 3 mm minimum thickness is generally considered to provide sufficient mechanical protection, except where shot-fired nails are likely to be used.

Yes, I know, and it's one of these figures that appears all over the place - which begs the question of where it comes from. Specifically, as I implied, it is important in deciding whether use of products such as 'Safeplates' is compliant with BS7671. What is your view, and that of others?

Regulation number(s) 522.6.101 522.6.103

... and, as I said, 522.6.100 - which relates to the specific issue we're discussing here (cables in notched joists). The two mentioned by ESC relate to the corresponding issue in walls.

Kind Regards, John

 

[GaryMo]

Not that I've tried but if I took a 1mm thick 'Safeplate' and hammered a nail straight through it then I wouldn't be happy to state that an installation is compliant with BS7671 where they've been used to protect cables at a depth of less than 50mm that don't have additional protection by a 30mA RCD.
Whether it's notched joists or embedded within walls, the logic is still the same.

Now, I'm pretty sure I wouldn't even need to attempt to hammer a nail through a 3mm thick piece of steel to know that I'd be unsuccessful.

 

[JohnW2]

Not that I've tried but if I took a 1mm thick 'Safeplate' and hammered a nail straight through it then I wouldn't be happy to state that an installation is compliant with BS7671 where they've been used to protect cables at a depth of less than 50mm that don't have additional protection by a 30mA RCD.

Well, I have just tried Using a standard hammer and a whole range of nails, whether I use 'reasonable' or 'unreasonable' force, I get nowhere near to penetrating a 'Safeplate' - and the same was true when I resorted to a club hammer. In all cases, it was the nail which suffered, with minimal effect on the 'Safeplate'!

Am I not right in saying that a lot of steel conduit only has a wall thickness of 1.5mm or 2mm? If so, do you feel that such conduit does not offer adequate protection to satisfy BS7671 (or your own 'happiness', as above)?

Kind Regards, John

 

[GaryMo]

Obviously a 1mm plate is less penetrative than I thought!

Thinking about a length of 20mm conduit with a wall thickness of 1.5mm, I'd never have thought I could hammer a standard nail through it.

Maybe 3mm is overkill but at the same time it's also pretty much a guarantee.

I've only ever resorted to covering cables via a steel plate within a wall once. That was for a garage conversion to a kitchen and for the main tails that ran up the wall towards a high level consumer unit. I used a manufactured 3mm thick steel plate and was happy that the 100A protected tails would remain intact from the kitchen fitter.

 

[JohnW2]

Obviously a 1mm plate is less penetrative than I thought!

So it seems - unless I'm a lot weaker than I thought! It obviously depends upon the type of steel. I could probably knock a nail through 1mm of some types, but they hopefully wouldn't make cable protectors out of them.

Thinking about a length of 20mm conduit with a wall thickness of 1.5mm, I'd never have thought I could hammer a standard nail through it.

Quite.

Maybe 3mm is overkill but at the same time it's also pretty much a guarantee.

Sure. 5m or 10mm would be an even more guaranteed guarantee! The question is what is 'reasonable' and 'adequate'. There is probably never a guarantee against the total idiot but, for the vast number of people, all one really needs is something 'unpenetrable enough' to make it clear to the user of a hammer/screwdriver/drill that (s)he has hit something other than plaster/masonary/wood/whatever.

I've only ever resorted to covering cables via a steel plate within a wall once. That was for a garage conversion to a kitchen and for the main tails that ran up the wall towards a high level consumer unit. I used a manufactured 3mm thick steel plate and was happy that the 100A protected tails would remain intact from the kitchen fitter.

As above, I reckon that's more than enough protection against even the worst of kitchen fitters. My interest (and the interest in this thread) is about notched floor joists. When I acquired my present (old) house, like many others I inherited an electrical (and plumbing) system almost entirely based on cables in notched joists. Over the years, when I've had access, I have gradually added protection - initially with 'ad hoc' bits of metal, more recently with commercial products like 'Safeplates' when they appeared on the scene. I'm personally perfectly happy with the degree of protection they offer, but just wondered (in view of all the "3mm" assertions) whether this degree of protection would actually be considered adequate to keep BS 7671 (hence Mr Jobsworth and his colleagues) happy!

Kind Regards, John

 

[RF Lighting]

When I was an appo and we were reusing old notches, we used to nail straight through sape plates to hold them in place when the little spikey things didn't hold them properly.

As for conduit, it's quite hard to penetrate, but not impossible, but even if you did penetrate it, the risk is minimal as the conduit is earthed and the OCPD would operate.

 

[RF Lighting]

https://www.youtube.com/watch?v=dFfIrTTFfFc

 

[JohnW2]

https://www.youtube.com/watch?v=dFfIrTTFfFc

Well, I imagine that the nail gun is probably more powerful than me and my hammer!

So, what do you conclude - given that a nail gun can apparently shoot nails through steel conduit, and the fact that most (if not all) standard steel conduit has a wall thickness less than this oft-quoted 3mm figure, do you feel that steel conduit does not afford enough protection to satisfy BS7671?

Kind Regards, John

 

[Monkeh]

It's pretty futile to try and protect a cable from a nailgun. Little finish nailers are too puny, anything for structural work.. no. You're not going to succeed, give up now. The damn things are designed to shoot through sheet metal.