Quite nervous doing this...

[TerraHertz]

On the topic of Aluminized insulating sheet (as in the photo), a few years back (2010) there was a big government scandal in Australia.
A famous singer, Peter Garrett of Midnight Oil, had been elected to Australian federal parliament and became minister for the environment. He then oversaw a government scheme to subsidize improvements in home energy efficiency, by installing thermal insulation. Mostly in roofs.
Great, but the problem was he ignored advice that one form of insulation being used was unsafe. Metalized foil...

Something like 93 house fires and four deaths later, Peter Garrett took the blame.

The danger lay in installation methods. Installers would roll the stuff out in attics, then use staple guns to fix it down to the ceiling joists. But typically the electrical wiring in ceilings is also fixed to the joists, and a person on top of the insulation can't see if he's going to be stapling into wiring. The wires are only PVC insulated, not metal conduit.

So anyway, turns out the metalization IS electrically conductive.

[Ed.Kloonk]

(...) Peter Garrett took the blame.

(...)

No he bloody well didn't.

[GlennSprigg]

'Legal' doesn't always mean 'Right'/'safe'...
Generally here in Australia, if power is supplied from Poles on the street, then on the poles,
they have main fuses on the pole for the premises, (or gutter riser), before the main S/B feed.
(Sometimes, these are underground from the street).
However, I've seen small factories where the street supply traverses inside the roof for say 50m
before being terminated in the main switchboard with supply fuses, on the other side of the building!
Meanwhile, there is unprotected "street fault current" on the supply through the roof!!!
(Sorry... I just remembered that)

[SeanB]

Street fault current is typically handled by the 400A per phase fuse on the secondary of the distribution transformer. If the transformer is bigger, and feeding more loads, then there will be a few outgoing cables at low voltage, each with it's own set of 400A fuses per phase. They take around 2 seconds to blow if there is a short at the far end of the cable, but can blow faster if you have somebody put a pickaxe through the cable closer to the transformer.

The cable to the factory would have to be rated for the fault current, so they will almost always also have a set of pole mounted fuses at the tap off point, even if the factory has fused disconnects inside by the meter. Might be 100A per phase inside, and 160A per phase outside, with the cable able to not fail before the fuse blows. Overhead cables can stretch quite a bit on overload, especially the bare copper ones on old installations, which are pretty well insulated by the thick copper oxide layer on them.

[Zero999]

That looks pretty dangerous and could be against the local code!

?!

Double insulated cabling, what's the problem?

The problem is what he has done. We are not addressing the cables in this post.

... and what is it he's done? Placed an object next to a cable?

Placing a metal/conductive object next to some cables is not dangerous, as long as the cables are double insulated, which they are in this case.

It seems like many people see cables laying on aluminium foil and panic about it being dangerous, without actually thinking about it. If it were unsafe to put aluminium foil under those cables, then they be exposed like that, so anyone can touch them. In reality there are no exposed, live conductors, so there's no additional risk of electrocution by putting aluminium foil there. You'll see a similar type of thing inside a double insulated, metal cased appliance, such as a DVD player.

[james_s]

On the topic of Aluminized insulating sheet (as in the photo), a few years back (2010) there was a big government scandal in Australia.
A famous singer, Peter Garrett of Midnight Oil, had been elected to Australian federal parliament and became minister for the environment. He then oversaw a government scheme to subsidize improvements in home energy efficiency, by installing thermal insulation. Mostly in roofs.
Great, but the problem was he ignored advice that one form of insulation being used was unsafe. Metalized foil...

Something like 93 house fires and four deaths later, Peter Garrett took the blame.

The danger lay in installation methods. Installers would roll the stuff out in attics, then use staple guns to fix it down to the ceiling joists. But typically the electrical wiring in ceilings is also fixed to the joists, and a person on top of the insulation can't see if he's going to be stapling into wiring. The wires are only PVC insulated, not metal conduit.

So anyway, turns out the metalization IS electrically conductive.

The wires are run across the top of the joists? Here they're run either along the side of a joist or if they need to cross they are run through a hole drilled through the middle. On the side the sheetrock is attached you have a nail plate that is stuck to the joist or stud to prevent a nail or screw from damaging the wire. Normally you wouldn't have anyone nailing or stapling things down into the top of joists in the attic here so I don't know what code says about that.

[ebastler]

It seems like many people see cables laying on aluminium foil and panic about it being dangerous, without actually thinking about it. If it were unsafe to put aluminium foil under those cables, then they be exposed like that, so anyone can touch them. In reality there are no exposed, live conductors, so there's no additional risk of electrocution by putting aluminium foil there. You'll see a similar type of thing inside a double insulated, metal cased appliance, such as a DVD player.

Well, the big difference in my mind is that if something should go wrong with the DVD player, I can pull its plug without having to step inside its metal case. 

Obviously everything is fine as long as the installation is in good shape, cable isolation intact etc.. But I can think of various situations where something goes wrong, which make me feel queasy: Insulation gets damaged; someone does a sloppy job, installs a live wire and drops it onto the metal floor; an earth or neutral wire touches the floor and grounds your feet really well when you unfortunately touch some live wire, etc.

Yes, all these scenarios are not very likely to occur. But since it is so easy to avoid, why put a conductive layer on the floor of your installation cabinet to make the potential consequences so much worse?

[paulca]

Yes, all these scenarios are not very likely to occur. But since it is so easy to avoid, why put a conductive layer on the floor of your installation cabinet to make the potential consequences so much worse?

You know that a lot of office buildings have metal panel flooring right?  The hung ceiling buildings usually have a gap under the floor and above the ceiling tiles for running cables.  The ceilings are fibre board squares and the floors are metal plates set into a framework.  Usually plug sockets and network end points are presented in special boxes placed in such metal plates.


It allows floors to be completely stripped back and completely rewired and rebuilt without having to do any actual building work.

Anyway, as a test I took a piece of this flooring and put my meter on it.  OL.  So I punctured the metal foil layer with the probes and got... OL.  I tested the meter, it works.  So while the extremely thin layer of metalisation forming the vapour barrier might be conductive it seems really difficult to actually make a connection to it as the plastic layer lines the puncture.

Even if I wanted to Earth the flooring I don't know how I'm meant to make a connection to it.

EDIT:  I suppose a real test would be puncturing it with mains voltage, instead of the meesly little voltage my meter produces on Ohms... but I'm not prepared to do that.  Might be one for ElectroBoom on YT.

[Marck]

My bench is covered with a metallic looking vinyl that I can strip off and recover when ever it gets a bit ratty.  I had similar thoughts about conductivity and it carries a much higher risk than a floor in a cupboard with a cable or 2 above it.

So I borrowed an insulation tester and put 1000 volts across it.  No sign of conductivity and testing at 4X mains voltage gave  reassurance

In this case I would leave the material you have put down and use a good quality tape to seal the edges mostly as a moisture barrier and then get a bit of carpet cut it to shape and lay that over the top for a barrier to protect that insulation and as another layer of insulation.

But I do agree that the electrocution risk in this instance is not to bad and I wouldn’t loose any sleep over it.  But I would sell a kidney to stop a freezing draft so finding a way to stop that would be on the top of my list. 
M

[ebastler]

You know that a lot of office buildings have metal panel flooring right?  The hung ceiling buildings usually have a gap under the floor and above the ceiling tiles for running cables.  The ceilings are fibre board squares and the floors are metal plates set into a framework.  Usually plug sockets and network end points are presented in special boxes placed in such metal plates.

It allows floors to be completely stripped back and completely rewired and rebuilt without having to do any actual building work.

Hung ceilings are pretty much standard in office buildings here. But I don't think I have come across any raised floors except in a few special-purpose rooms (server/computer rooms). And where raised flooring is installed, I would typically expect the actual tiles to be made of, or at least covered with, a material that is non-conducting or maybe static-dissipating, not plain metal.

[paulca]

I would typically expect the actual tiles to be made of, or at least covered with, a material that is non-conducting or maybe static-dissipating, not plain metal.

The buildings I have worked in with this type of floor are IT office buildings.  The metal plates are covered with anti-static carpet, but it comes loose sometimes which is why I know the panels are metal underneath.

[BradC]

Generally here in Australia, if power is supplied from Poles on the street, then on the poles,
they have main fuses on the pole for the premises, (or gutter riser), before the main S/B feed.
(Sometimes, these are underground from the street).

Dunno which side of Aus you are on, but over in the West it's overhead straight from the pole (16mm x 4 on this place) to the point of attachment where it connects to a 10mm x 4 TPS to the switch board which then terminates in the 32A service fuses. Yes, that whole run is subject to the transformer fault current if it goes bang. There are no individual pole mounted fuses, just the drop-outs per segment.

There are different configurations for the underground stuff, but we are still predominantly overhead.

I've actually seen a set of drop-outs respond to a catastrophic fault current and there was no noticeable delay (tree branch shorted 2 phases). I was straight over the road from the pole at the time, looking up at the configuration as we'd lost a phase. Needed to go home and change my undies.

[Jeroen3]

I have a thin reflective insulating foil under my floor. The hatch to get to it is covered in flammability warnings.
It is also conductive. So you've made a large arc flash accelerator, cool...

[jmelson]

YIKES!!!  In the US, all the power entry cables are in metal conduit, or buried and armored.  Only the lower current cable (we call it "Romex") is allowed to be exposed.  So, there's a breaker on the transformer,  then buried cable to the meter, then a service entry breaker right after the meter, then into the main panel with another main breaker there.  Up to this point it is all encased in
some kind of steel protection.

Jon

[james_s]

PVC is allowed, at least by the NEC, at least it was the last time I did a panel replacement. There may be some localities that require steel conduit though.

[TerraHertz]

(...) Peter Garrett took the blame.

(...)

No he bloody well didn't.

Sorry, imprecise. I meant 'took the blame' as in he got sacked because of it. I wasn't paying attention at the time and don't recall if he ever commented that he could have done better, or 'not my fault'. Perhaps I should have said "took the fall."

The wires are run across the top of the joists? Here they're run either along the side of a joist or if they need to cross they are run through a hole drilled through the middle. On the side the sheetrock is attached you have a nail plate that is stuck to the joist or stud to prevent a nail or screw from damaging the wire. Normally you wouldn't have anyone nailing or stapling things down into the top of joists in the attic here so I don't know what code says about that.

Even in my new house built in 2000, mains wiring in the roof space is just run randomly via shortest path possible. Rarely if ever fastened down, and just lies across the tops of the ceiling beams.  Which is fine, unless you place something opaque and electrically conductive over it all, then start punching staples down through that.

I recall at the time of that scandal, thinking I bet Peter Garret never got up in a roof space and did any work in there.

[Ed.Kloonk]

(...) Peter Garrett took the blame.

(...)

No he bloody well didn't.

Sorry, imprecise. I meant 'took the blame' as in he got sacked because of it. I wasn't paying attention at the time and don't recall if he ever commented that he could have done better, or 'not my fault'. Perhaps I should have said "took the fall."

The wires are run across the top of the joists? Here they're run either along the side of a joist or if they need to cross they are run through a hole drilled through the middle. On the side the sheetrock is attached you have a nail plate that is stuck to the joist or stud to prevent a nail or screw from damaging the wire. Normally you wouldn't have anyone nailing or stapling things down into the top of joists in the attic here so I don't know what code says about that.

Even in my new house built in 2000, mains wiring in the roof space is just run randomly via shortest path possible. Rarely if ever fastened down, and just lies across the tops of the ceiling beams.  Which is fine, unless you place something opaque and electrically conductive over it all, then start punching staples down through that.

I recall at the time of that scandal, thinking I bet Peter Garret never got up in a roof space and did any work in there.

All good mate. It's that higher-than-thou prick PG that I love to hate.

Wires in the roof are supposed to be clipped. My understanding is the gyprocker should be able to come though after the rough-in and not have to deal with any cables to wrench on or pinch or whatever. You can get a blister if an inspector decides to check in the roof.

[aargee]

Not that I’m a friend of politicians but the reality was that the whole insulation debacle was rogue operators that did not adhere to standards for insulation, they should have been the ones that were pursued for the problems they created.

[GlennSprigg]

Street fault current is typically handled by the 400A per phase fuse on the secondary of the distribution transformer. If the transformer is bigger, and feeding more loads, then there will be a few outgoing cables at low voltage, each with it's own set of 400A fuses per phase. They take around 2 seconds to blow if there is a short at the far end of the cable, but can blow faster if you have somebody put a pickaxe through the cable closer to the transformer.

The cable to the factory would have to be rated for the fault current, so they will almost always also have a set of pole mounted fuses at the tap off point, even if the factory has fused disconnects inside by the meter. Might be 100A per phase inside, and 160A per phase outside, with the cable able to not fail before the fuse blows. Overhead cables can stretch quite a bit on overload, especially the bare copper ones on old installations, which are pretty well insulated by the thick copper oxide layer on them.

Thank you for your reply!!!
However, I can assure you that there were NO supply fuses on the pole. (Their fuses are at the main board).
There may have been say 3-phase 4-core 10 or 16 mm cables through the roof, that are unratingly protected.