Attic Insulation and Electrics

druidhill

Hi,

planning to insulate the attic to a good standard (there is only polystyrene pieces at the moment, very poor indeed) and I am concerned about the electrical cables in relation to this.

In relation to electrics, the house was built in the 1950s or so and the cabling seems to be black and circular (as opposed to the flatter, wider grey cable, although this was used at some point when a couple of extra sockets were fitted in a couple of rooms). The wiring seems to be running over the joists and down through the ceiling wherever required (using the shortest path / least expense principle, although to be fair that would be normal back then). I can try and provide more info on this, if you have any specific questions.

I have seen insulation done in houses (including by those registered for those government grant schemes) where the insulation is just rolled over the wiring.

What is the CORRECT and proper way to insulate when dealing with wiring? I would love to hear the advice of qualified electricians in particular, as safety is a big concern for me, and it's been the thing that has held me back the last few years, re. attic insulation.

Also, re-wiring of the house is not really an option (in terms of cost and also the electrics in the house has never been a problem) that I plan to consider.

TIA,
druidhill

Avns1s

If safety is a big concern for you as you say, then I'd start with rewiring. If the existign wiring is as old as you say and as you described, then I would be worried that it is the old rubber insulated wiring and that is a hazard.

I know I am not answering your question, but I think the priority probably should be the wiring and I think, putting insulation on top of rubber insulated wiring, would be a big mistake.

the_syco

Avns1s wrote: »

If safety is a big concern for you as you say, then I'd start with rewiring. If the existign wiring is as old as you say and as you described, then I would be worried that it is the old rubber insulated wiring and that is a hazard.

Two reasons for rewiring:
a) as said above
b) if it's as you describe, there may may be a few bits of wire that you may not see.

druidhill

Thanks for reply, I'm not 100% sure if it is rubber wiring (or an older type of PVC cable).
At the moment, the best I can afford to do is insulate between the joists (not over), and there should be enough 'play' in the wiring for this where the wiring will be on top of the insulation for the most part. The problem I am unsure about is how to effectively insulate where the wiring goes between the joists and into the ceiling board.

I certainly would not be happy insulating over the joists with the existing wiring.

evosteo

a rewire is expensive but look you could think of it as an investment

put money away over time and make a point of addressing asap

you can get a house rewired from €2500 now adays depending on size and spec

that old cabling is a nightmare cause if anything goes wromg with it most electricians wouldnt touch it with a barge poll,

if its located in the vicinity of any heat source ie rad pipes then the insulation on that cable WILL cruble when touched, iv seen this with my own eyes

on the insulation

when installin it try not to force any of the cables, connection could be loose and they may come out of connections

try lay the cable above the insulation where possible

if the cable is travellin through a joist, cut the insulation at this point and continue it after the cable, the cable should then be nestled between the insulation

paddy147

A picture can be helpfull too.

Cables ontop of insulation.:)

druidhill

paddy147 wrote: »

A picture can be helpfull too.

Cables ontop of insulation.:)

Yup, think that's the way I'll go for now as the best option for the moment in terms of benefit vs. cost (love to be able to re-wire but don't have the funds).

In relation to safety with insulation and electrical wiring, I remember an electrician (old school/now retired) saying a few years back that it shouldn't really be an issue as the fuse(s) in the fusebox would be the weakest link if there was a concern about overheating - this may be an unacceptable comment by today's standards.

paddymick

druidhill wrote: »

In relation to safety with insulation and electrical wiring, I remember an electrician (old school/now retired) saying a few years back that it shouldn't really be an issue as the fuse(s) in the fusebox would be the weakest link if there was a concern about overheating - this may be an unacceptable comment by today's standards.

I`d agree with that I think if cables get to the stage where they become hot and dangerous then they havent been fused right.

On the rewire thing anytime I come across a job were I`d recommend a rewire and they say it would be to expensive I offer them a cheaper alternative..........A couple of smoke alarms;)

NewHillel

paddymick wrote: »

I`d agree with that I think if cables get to the stage where they become hot and dangerous then they havent been fused right.

On the rewire thing anytime I come across a job were I`d recommend a rewire and they say it would be to expensive I offer them a cheaper alternative..........A couple of smoke alarms;)

That's not good advice, poor wiring can cause death - not just from fire..
There is some black wiring that is "just about" acceptable. Most is a electrical safety and fire hazard. Only an experienced tradesman, or engineer, could tell the difference.

The advice to he OP was sound - get the wiring sorted, first. Even fitting new insulation could cause a real problem as it could impair old insulation.

paddymick

NewHillel wrote: »

That's not good advice

Which part was the bad advice?? I can only assume you are referring to smoke alarms as an alternative for a rewire and if so then you are obviously a bit slow on the take up of what I said which is basicly that if you dont get your house rewired you will need some smoke alarms as you are at a very high risk of fire due to old and dangerouse wiring!!

NewHillel

paddymick wrote: »

Which part was the bad advice?? I can only assume you are referring to smoke alarms as an alternative for a rewire and if so then you are obviously a bit slow on the take up of what I said which is basicly that if you dont get your house rewired you will need some smoke alarms as you are at a very high risk of fire due to old and dangerouse wiring!!

I'm not slow, at all - and no need to get personal.
If that was what you meant, well and good.

paddymick wrote: »

On the rewire thing anytime I come across a job were I`d recommend a rewire and they say it would be to expensive I offer them a cheaper alternative..........A couple of smoke alarms;)

Bruthal

NewHillel wrote: »

I'm not slow, at all - and no need to get personal.
If that was what you meant, well and good.

It seemed obvious enough that he was saying there is more of a risk of fire if bad wiring installations are not upgraded properly.

Bruthal

NewHillel wrote: »

That's not good advice, poor wiring can cause death - not just from fire..

That taps becoming live thing is possible in a properly wired installation too, when the installations are neutralized. A broken utility supply neutral can cause it.

NewHillel

robbie7730 wrote: »

That taps becoming live thing is possible in a properly wired installation too, when the installations are neutralized. A broken utility supply neutral can cause it.

For that very reason I've earthed all pipework in my house directly, even though it wasn't when I bought it.

M cebee

NewHillel wrote: »

robbie7730 wrote: »

That taps becoming live thing is possible in a properly wired installation too, when the installations are neutralized. A broken utility supply neutral can cause it.

For that very reason I've earthed all pipework in my house directly, even though it wasn't when I bought it.[/Quotes]

so you're an experienced engineer or tradesman then

Bruthal

NewHillel wrote: »

For that very reason I've earthed all pipework in my house directly, even though it wasn't when I bought it.

As i said though, in a properly wired neutralized house, with all pipes earthed, if the main neutral fails upstream of the neutralized point(outside house), all earthed items in the house can approach 230v. The only thing that is now connecting all earthed items in the house to ground, is the earth rod.

Since the installation now has a broken neutral at 230v directly connected to all earthing in the house, the earthing will be at or close to 230v.

If its all properly earthed, all items will be at the same potential to help reduce shock risk. But taps would likely be at near phase voltage, since a single earth rod is unlikely to keep the neutral at ground/neutral potential, especially if any substantial loads were on when the neutral failed.

In the case where the fatal shock happened, it was possible just one tap became live. That would be highly dangerous alright with an earthed one beside it.

whizbang

(there is only polystyrene pieces at the moment

If the wiring has come into contact with the polystyrene at any point then wiring has to come out.

as per pic, you can put any amount of rockwool/glassfibre insulation over cables, but not within a mile of polystyrene.

NewHillel

robbie7730 wrote: »

If its all properly earthed, all items will be at the same potential to help reduce shock risk. But taps would likely be at near phase voltage, since a single earth rod is unlikely to keep the neutral at ground/neutral potential, especially if any substantial loads were on when the neutral failed.

In the case where the fatal shock happened, it was possible just one tap became live. That would be highly dangerous alright with an earthed one beside it.

If its all properly earthed, all items will be at the same potential - ground.

Above is not true, I was wrong !

Bruthal

NewHillel wrote: »

If its all properly earthed, all items will be at the same potential - ground.

No, thats simply not true. But your not the first on here to claim it is. All metal in the house will be at the same potential alright, but not at ground potential if decent load is on in the house.

If neutral breaks out side, the house has one single connection to the ground, and this is the earth rod, because the broken neutral has left the house earth rod isolated from all other earth rods on the local network.

Anyone with some experience of broken neutrals will realise a single earth rod at best will not be a good solid link to earth. Im guessing you think it is.

Well if it is a good solid connection to the ground, this means the house appliances will all function as normal. But i can tell you that is not the case. I have been at a few houses where neutral has failed outside, and nothing in the house will work right, apart maybe from a low wattage bulb or 2.

I have been at one where the main neutral onto the mini-pillar failed, leaving several houses in a star configuration with no neutral. This scenario would of had multiple earth rods, at the pillar neutral, and at each affected house, all still linked as their concentric neutrals were all still connected to the pillar.

But the neutral still went well above earth potential because of the imbalance in the house loads. The house i got called to had 350v L to N. So The neutral was well away from ground potential.

As well as this, the reason for neutralised installations requiring all extraneous metal be bonded, is for the very reason, that in neutral failure upstream of neutralizing point, that all metal will be at equal potential, simply because its going to go well above ground potential.

This notion that the single earth rod will keep it solidly at ground potential is just not true. If it did do this, the broken neutral would go un-noticed.

M cebee

only an experienced engineer can know for sure
wah!

Bruthal

M cebee wrote: »

only an experienced engineer can know for sure
wah!

Or a carpenter like myself:eek:

fodda

When insulating can you put plastic waste pipe over the wiring by splitting it and taping back up so this then becomes a conduit and protects or allows air around the wiring to cool it ........and then cover with insulation?

Bruthal

fodda wrote: »

When insulating can you put plastic waste pipe over the wiring by splitting it and taping back up so this then becomes a conduit and protects or allows air around the wiring to cool it ........and then cover with insulation?

No need to do that when using fibre glass insulation.

fodda

robbie7730 wrote: »

No need to do that when using fibre glass insulation.

I thought you couldn't put insulation including fiberglass insulation directly over wiring because it could get hot when heavy voltage was being used?

Bruthal

fodda wrote: »

I thought you couldn't put insulation including fiberglass insulation directly over wiring because it could get hot when heavy voltage was being used?

Properly sized/protected cables with the correct MCB`s wont get very hot. Maybe others here will differ, but i wouldnt of thought it was a problem in fibreglass. Multiple circuits are often run in trunking in commercial installations, and these would be more confined than single cables in the attic.

m cebee will be along soon to give the correct answer though:)

2011

robbie7730 wrote: »

Properly sized/protected cables with the correct MCB`s wont get very hot. Maybe others here will differ, but i wouldnt of thought it was a problem in fibreglass.

+1

If the cable runs were very long with large loads the cable size should be increased. For the average house it would not make any difference.

If in doubt the cable size can be calculated using the regs. A de-rating factor can be applied for the insulation. There is also a grouping factor for multiple circuits in a trunking.

2011

robbie7730 wrote: »

As i said though, in a properly wired neutralized house, with all pipes earthed, if the main neutral fails upstream of the neutralized point(outside house), all earthed items in the house can approach 230v

Correct.
How close extrenious metalwork gets to 230V depends on a number of factors, including size of connected loads and impedance of earth rod connection.

2011

NewHillel wrote: »

If its all properly earthed, all items will be at the same potential - ground.

Between any point on the metalwork and the star point of the transformer there will be a certain impedance. If the neutral is broken a current will flow through the metalwork and earth cabling. Ohms law will then indicate that a potential difference will then exist between the star point of the traffo and the metalwork (V=I x Z). As most of the impedance (between exposed metal and star point) exist between the ground and the metalwork the largest volt drop will be present there too.

NewHillel

robbie7730 wrote: »

As i said though, in a properly wired neutralized house, with all pipes earthed, if the main neutral fails upstream of the neutralized point(outside house), all earthed items in the house can approach 230v. The only thing that is now connecting all earthed items in the house to ground, is the earth rod.

Since the installation now has a broken neutral at 230v directly connected to all earthing in the house, the earthing will be at or close to 230v.

I accept that the earth potential could rise, slightly, if the main neutral fails upstream. However, I cannot envisage any circumstances, other than a badly installed or faulty local earthing system, where the earth potential in a domestic installation could be at, or even close, to 230v. That's entirely leaving aside the fact that the RCCB should trip out.

Don't take my word for it. From the FAS training notes: "The function of the earth electrode is to maintain a connection between the general mass of earth and metallic parts of the consumer’s installation. These can then be regarded as being at zero potential. The earth electrode must be continuously effective and capable of carrying earth leakage and earth fault currents, which may arise."

NewHillel

2011 wrote: »

Between any point on the metalwork and the star point of the transformer there will be a certain impedance. If the neutral is broken a current will flow through the metalwork and earth cabling. Ohms law will then indicate that a potential difference will then exist between the star point of the traffo and the metalwork (V=I x Z). As most of the impedance will exist between the ground and the metalwork the largest volt drop will be present there too.

Why do you think that most of the impedance will exist between the ground and the metalwork?

Bruthal

What actually happens is the load current only has a path down the earth rod, although it will only be a fraction of what the load current would be on an intact neutral.

So a 3kw 18 ohm load in series with an 18 ohm(example) earth rod will have 115v on the neutral above ground potential. And as the earthing system is connected to this, all earthed metal in the house will also be at this voltage.

In practice a single earth rod impedence to earth may be a fair bit higher than this, especially in dry weather. An earth rod at a neutralized house is not just an earth rod for that house alone, but is 1 earth rod of many, all connected together through the neutral conductors from the transformer.

Bruthal

NewHillel wrote: »

I accept that the earth potential could rise, slightly, if the main neutral fails upstream. However, I cannot envisage any circumstances, other than a badly installed or faulty local earthing system, where the earth potential in a domestic installation could be at, or even close, to 230v. That's entirely leaving aside the fact that the RCCB should trip out.

Don't take my word for it. From the FAS training notes: "The function of the earth electrode is to maintain a connection between the general mass of earth and metallic parts of the consumer’s installation. These can then be regarded as being at zero potential. The earth electrode must be continuously effective and capable of carrying earth leakage and earth fault currents, which may arise."

I dont need to take your word for it, or fas training notes. I have seen it in real life. You are assuming from ideal and perfect scenarios, im talking about real life events tested with meters.

In a tt system, an RCD is used for ensuring a live to earth short trips the circuit MCB. Any idea why this RCD is required?

It is the same reason why neutralizing is used at all. Do your notes tell you why?

I have actually connected 10 amp MCBs direct to earth rods, and the MCBs didnt trip. Guess why?

Often, real life principles vary from text book expectations.

NewHillel

robbie7730 wrote: »

I dont need to take your word for it, or fas training notes. I have seen it in real life. You are assuming from ideal and perfect scenarios, im talking about real life events tested with meters.

I have not suggested that such voltages don't happen - I too have seen them in real life. However, they do not arise where there is a properly installed, and fully functioning earth system. (Last line is not correct.) I readily accept that there are many departures from this ideal - poor earth rod location, prolonged dry spell, earth rod deliberately cut short, oxidised connection etc...

(Oh, it used to be a common practice to wet the ground ner the earth rod, to ensure it passed the test. I have no idea whether that still goes on.)

Bruthal

NewHillel wrote: »

Why do you think that most of the impedance will exist between the ground and the metalwork?

You dont really believe an earth rod has 0 ohms ground connection do you?

I remember almost identical debate about this 3 years ago here, and it wasnt fas notes the poster was reading from.

A single earth rod will have quite a few ohms to ground impedence, and this often increases as a current flows down it to the ground. Current only flows to ground to return the the neutral as the neutral is earthed at multiple points.

Why multiple points? Becasue a single point does not make a low impedence connection.

Bruthal

NewHillel wrote: »

I have not suggested that such voltages don't happen - I too have seen them in real life. However, they do not arise where there is a properly installed, and fully functioning earth system. I readily accept that there are many departures from this ideal - poor earth rod location, prolonged dry spell, earth rod deliberately cut short, oxidised connection etc...

Your not right there. I know your not seeing it because you dont seem to have a solid grasp of it.

A fully functioning earth system is highly dependent on an intact neutral in the neutralized system. Do you understand that part?

If the neutral fails, a solitary earth rod is all that there is to keep the open neutral at ground potential. If it did keep it at earth potential, then the house would operate as normal, and the broken neutral would go un-noticed. It will stay at ground potential if an 11 watt cfl is all thats on. But try turning on the shower. The shower absolutely will not work, but if your earth rod kept the open neutral locked at ground potential, then the shower would work perfectly.

Now, again, the above is exactly why all metal work muxt be bonded in a neutralized installation, but you are not grasping the concept of why.

And also for the same reason, a tt system uses a main RCD covering the entire system, a time delay 300ma one i think they use. This is because the earth rod can not be relied upon to trip MCB`s in earth fault scenarios.

Read this, page 9, section 5.1.2 paragraph ii, carefully. It might say page 13 on the reader, but page 9 of the actual pages themselves. But it can be found from the 5.1.2 anyway

NewHillel

robbie7730 wrote: »

Your not right there. I know your not seeing it because you dont seem to have a solid grasp of it...

You're right, I didn't!

That's me in my box.

Thank you, and goodnight!

Bruthal

NewHillel wrote: »

You're right, I didn't!

That's me in my box.

Thank you, and goodnight!

Well i didnt mean it in a bad way, maybe you are seeing it, and im not:)

cabledude

Ah lads the poor misfortune who submitted the original post has just been bombarded with technical mumbo jumbo : impedance/ ohms / grounding. All well meaning of course but I'd imagine he/she is well lost at this stage.

Getting back to the content of the original post :

If you have rubber insulated energy cables in your home I would strongly advise you to have them checked by a competent electrician. There was a very good reason for the cable manufacturers moving to PVC/LSF sheathing plastic's. They are more durable and safer. I'm not sure how rubber compounds would live alongside insulation. Especially if they were installed 50 years ago. The process used to bind the rubber around the cores(red/black wires) is inferior compared to extruded plastic. I've no idea how well this process would hold together after 50 years. This could be dangerous. Rubber over time will deteriorate. I don't mean to alarm you but if it were my home I'd get them replaced, or at the very least, checked out......

2011

NewHillel wrote: »

Why do you think that most of the impedance will exist between the ground and the metalwork?

Most of the impedance is made up of resistance.

The formula for resistance is:

R = ρl / a

Where:
R = resistance
ρ = resistivity
l = lenght
a = cross sectional area

If you look at the resistance of the general mass of earth you will find that "a" is quite large! This makes R quite small.

Bruthal

2011 wrote: »

If you look at the resistance of the general mass of earth you will find that "a" is quite large! This makes R quite small.

Yea thats the part many do quote, but they forget that a single earth rod, or even a few, dont have a large "a" in contact with the ground.

Also, the impedence measured with a tester, even if low, is unlikely to be reflected for long when a decent current flows through the solitary earth rod.

2011

robbie7730 wrote: »

Yea thats the part many do quote, but they forget that a single earth rod, or even a few, dont have a large "a" in contact with the ground.

I didnt forget. In fact it makes my point as this is part of the circuit between the general mass of earth and the metalwork.

Bruthal

2011 wrote: »

I didnt forget. In fact it makes my point as this is part of the circuit between the general mass of earth and the metalwork.

A now, was not suggesting you forgot, i was agreeing with your post, obviously you dont forget, from your previous posts about it in the thread.

But some only mention the large mass of earth to claim the earth rod path is as good as the neutral path. Which is total nonsense, unless an enormous earthing grid is installed.

NewHillel

2011 wrote: »

Most of the impedance is made up of resistance.

The formula for resistance is:

R = ρl / a

Where:
R = resistance
ρ = resistivity
l = lenght
a = cross sectional area

If you look at the resistance of the general mass of earth you will find that "a" is quite large! This makes R quite small.

Ah Cmon, now, I 'got' it, last night. (Honestly )

Thing is, I never gave it a second thought. Proper earthing giving adequate protection was something I just 'knew'. Its not something I work with it day-to-day. This topic really deserves a sticky thread, all on its own. I'm certain that many tradesmen who should know this, and the implications, don't.