Building regulations questions.

[ppelec100]

So I've just been reading through the electricians guide to the building regulations for my own knowledge and have come across 2 regulations which I need help on clarifying...

page 80 5.5.2 (e) mentions cables should need to withstand a temperature of 170 degrees C.
Q. now why on earth would a cable need to withstand that temperature if a human cannot even with stand that temperature? Also what kind of cable would that be then? When I checked the specs / data sheet of fp200 it only comes up with operating temp which is 70.

page 82 5.7.3 supplies to outbuildings mentions if an armoured cables supplies an outbuilding such as a shed or garage, care must be taken to only connected the armouring is not connected to the outbuilding terminals, it must be connected to the MAIN buildings earthing terminal.
Q. so what do you do with the armouring of the swa when on the outbuilding side? it seams like bad practice not to earth it to the fuse board. Does that also mean you just run a 2 core swa rather than a 3 core swa for single phase? as surely they wont want the earthing from the main building to connect with the outbuilding? are you supposed to provide the earthing only by a TT earth rod ground installation only?

I am still learning so don't kane me lol

Thanks for any answers!

 

[Micktw68]

I Cant help with point A, but as far as I can ascertain the reason for not connecting the Armour/CPC to the remote end of an external building is the prevention of introducing a potential difference with the loss of The N on a PME supply. The idea being the armour is isolated from the remote CU , which is earthed using its own TT. The Earthing at the main CU is purely for the submain supply cable .

 

[Deleted account]

To add to the @Micktw68 post above.

in most cases you can have the shed or garage on the same earthing system as the house so no potential difference issues arise.

it’s a common misconception that it must be TT ed.

as for the temperature issue. 170 degrees is hot for humans but not for everyday objects.
if a human goes to 170degree the tissue would be damaged beyond repair. If a cable got to 170 degrees it would not be damaged and would be capable of continued use.

 

[pc1966]

page 80 5.5.2 (e) mentions cables should need to withstand a temperature of 170 degrees C.
Q. now why on earth would a cable need to withstand that temperature if a human cannot even with stand that temperature? Also what kind of cable would that be then? When I checked the specs / data sheet of fp200 it only comes up with operating temp which is 70.

I don't know the building regulations, but for fault-survival (adiabatic limit) cables are often assumed to have upper limits for conductor temperature in short-term events (less than 5s) of 160C for PVC and 250C for XLPE for the final temperature after a fault has been cleared.

That most certainly not the working temperature!

Also you can have silicone rubber cables rated to work at 185C for special cases but that is unlikely in domestic cases, though would suit boiler and immersion heater supplies that might work above the typical 70C limit of PVC.

page 82 5.7.3 supplies to outbuildings mentions if an armoured cables supplies an outbuilding such as a shed or garage, care must be taken to only connected the armouring is not connected to the outbuilding terminals, it must be connected to the MAIN buildings earthing terminal.
Q. so what do you do with the armouring of the swa when on the outbuilding side? it seams like bad practice not to earth it to the fuse board. Does that also mean you just run a 2 core swa rather than a 3 core swa for single phase? as surely they wont want the earthing from the main building to connect with the outbuilding? are you supposed to provide the earthing only by a TT earth rod ground installation only?

You will find many threads on this forum covering out-building supplies and there is no one simple answer as it depends on the details of the house supply and what is in the out-building. But a short run-down is:

• The SWA armour must always be earthed. This should be via a proper SWA gland and not a pipe clamp! Most commonly this is done at the supply end at the very least.
• If the out-building has any extraneous conductive parts, for example metal service pipes (water, gas, oil feed, etc) or it is a steel structure and the main supply is PME (i.e. TN-C-S) then the minimum CPC for bonding is 10mm copper or the equivalent in steel armour (around 80mm CSA).
• As that is greater than most feed cables, you might have to go up to 10mm 3-core to use one of the cores as the CPC to meet the bonding requirements. NOTE: this bond size is independent of the max current the out-building needs, as it covers PME fault cases where you may have tens of amps for long periods of time going via the CPC to true Earth.
• Alternatively you TT the out building, in which case at the out-building you isolate the supply cable's CPC (e.g. using a plastic IP68 style of gland) and provide a separate earth rod for the local earth, and obviously you have to have ALL circuits on RCD protection out there.

Safety trade off is less obvious. With a TN supply (i.e. connecting main house MET to out-building CPCs) you get reliable disconnection on the OCPD (e.g. thermal-magnetic trip) if, as assumed here, the Zs values are low enough. This is reliable, but you have the risk of someone outside being exposed to elevated voltages on any bonded metalwork during a PME fault.

Going TT avoids the PME risk, but then you become dependent on the electronics of the RCD for any fault to earth which is more complex and less reliable than OCPD (i.e. the RCD is no longer "additional protection" as it is your primary protection for such faults).

 

[pc1966]

Just to add these days most outdoor power tools are either battery, so supply unimportant, or class II so not earthed anyway, so the "PME risk" is limited to things that might allow good contact to a person and the surrounding true Earth potential at the same time. Examples are hot-tubs, maybe fence railings, etc, that have class 1 flood lights fitted, etc.

Car chargers (EV) are the most obvious special case these days, but are being covered by additional regulations and fancier protection electronics in many cases due to the cost, trouble, and infrastructure risk of hammering in earth rods in built up areas.

Also the PME risk is probably a lot less than the risk of a fault in the installation, though it happens about 300 times a year over the whole UK I think (someone posted stats a while ago, seem to remember 300-ish)

 

[ppelec100]

I Cant help with point A, but as far as I can ascertain the reason for not connecting the Armour/CPC to the remote end of an external building is the prevention of introducing a potential difference with the loss of The N on a PME supply. The idea being the armour is isolated from the remote CU , which is earthed using its own TT. The Earthing at the main CU is purely for the submain supply cable .

So if the main buildings supply system was TN-S, the earthing will be be supplied SEPARATELY from the neutral conductor. It will be provided from its own armouring back to the substation? which means there would be no potential difference if the N conductor went out of service from TN-S system?

I Cant help with point A, but as far as I can ascertain the reason for not connecting the Armour/CPC to the remote end of an external building is the prevention of introducing a potential difference with the loss of The N on a PME supply. The idea being the armour is isolated from the remote CU , which is earthed using its own TT. The Earthing at the main CU is purely for the submain supply cable .

soo does that mean for a TNS supply the earthing would be connected to the outbuildings fuse board? As the earthing is provided directly through the outer armouring of the main building incoming supply cable...

Also what if there was a loss of the supply from a tn-c-s system, what about the rest of the main buildings installation earthing system? It would go out, right? Causing a potential difference?

 

[ppelec100]

To add to the @Micktw68 post above.

in most cases you can have the shed or garage on the same earthing system as the house so no potential difference issues arise.

it’s a common misconception that it must be TT ed.

as for the temperature issue. 170 degrees is hot for humans but not for everyday objects.
if a human goes to 170degree the tissue would be damaged beyond repair. If a cable got to 170 degrees it would not be damaged and would be capable of continued use.

To add to the @Micktw68 post above.

in most cases you can have the shed or garage on the same earthing system as the house so no potential difference issues arise.

it’s a common misconception that it must be TT ed.

as for the temperature issue. 170 degrees is hot for humans but not for everyday objects.
if a human goes to 170degree the tissue would be damaged beyond repair. If a cable got to 170 degrees it would not be damaged and would be capable of continued use.

So it is safe to use the armouring of the earth connecting to both main building and outbuilding? On a tn-s and tn-c-s System? I would Of thought so as the tn-c-s system has earthing points buried into the ground at intervals of the supply side going back to the substation, am I right? Therefore the earthing is not cut off if the neutral conductor fails?

so for the cables, what am I looking for when the cables mention temperature? All of them tend to say operating temperature At 70 degrees including fp200? But I’m pretty sure operating temperature and withstanding temperature are difference. Fp200 is capable of direct fire which is more than 70 degrees...

 

[snowhead]

as the tn-c-s system has earthing points buried into the ground at intervals of the supply side going back to the substation, am I right? Therefore the earthing is not cut off if the neutral conductor fails?

The Neutral conductor can and does fail after the earth spikes in the network, as happened on the small housing estate my Son lived on.

 

[telectrix]

if the supply neutral fails on TNC_S you will
1.lose your earth (might have some sort of earth through the bonding to gas and water).
2. any neutral cables where there is a load connected will be at 240V.

 

[pc1966]

So if the main buildings supply system was TN-S, the earthing will be be supplied SEPARATELY from the neutral conductor. It will be provided from its own armouring back to the substation? which means there would be no potential difference if the N conductor went out of service from TN-S system?

In the ideal TN-S case, yes. A fault on neutral has no impact on the E connections.

However, in reality most DNO networks are not "pure" with only an ideal case, so what looks to be TN-S at the home cut-out could actually be TN-C at some point between there and the transformer (where ideally the N-E common link is located).

soo does that mean for a TNS supply the earthing would be connected to the outbuildings fuse board? As the earthing is provided directly through the outer armouring of the main building incoming supply cable...

In most cases the best route is to connect the earth/CPC together, as most out buildings won't have extraneous conductive parts to worry about and most will not present a risk even if there is a PME fault.

The armour would then be glanded at both ends for a good connection. But you might still need to use 3-core for a copper CPC, or to have an external CPC run along side the SWA (might be cheaper, not a good approach unless in duct), for the following reasons:

• If PME and extraneous, to meet the 10mm copper equivalent minimum
• To meet the Zs for sub-main and/or final circuits disconnection times

You need to get to around 70/50/35mm for 2/3/4-core for the SWA armour to be "10mm copper" equivalent, and for even moderate lengths of typical sizes (like 4-10mm for modest loads/medium runs) you can find the sub-main R2 is too high for acceptable disconnection on the armour alone.

Also what if there was a loss of the supply from a tn-c-s system, what about the rest of the main buildings installation earthing system? It would go out, right? Causing a potential difference?

The main home also has the PME risk, but it should be adequately bonded for water/gas pipes, etc. With a 60-100A main supply and incoming cables of 16-25mm you would not really look at less than 16mm for main earth and 10mm for main bonds anyway.

The PME risk is really about voltage differences. Inside a home with adequate bonding you don't see the "true Earth" to get a shock. Outside where you have access to the CPC and the Earth you have that problem.

So it is not that the main home is intrinsically safe and an outbuilding unsafe, but more likely an outbuilding will be used for outdoor stuff, hence the greater concerns.

 

[pc1966]

So it is safe to use the armouring of the earth connecting to both main building and outbuilding? On a tn-s and tn-c-s

As above, in most cases it is the best approach, but you need to check for bond size minimum and anything like a hot-tub on the ground that really ought not to be on PME.

System? I would Of thought so as the tn-c-s system has earthing points buried into the ground at intervals of the supply side going back to the substation, am I right? Therefore the earthing is not cut off if the neutral conductor fails?

It depends on where it fails. Also even with a few earth spikes still present it might be a couple of ohms to true Earth. If the neutral current is tens of amps you might still be seeing over 50V there so not safe, even if not as bad as the full 230V from a complete open & no earth.

 

[DPG]

The Neutral conductor can and does fail after the earth spikes in the network, as happened on the small housing estate my Son lived on.

Happened at least twice on our estate. Once on the feed to my house. Made the metal clad light switch on the garage interesting to operate! Luckily they came out and fixed it quickly. Well actually they budged it a bit first - a bit of concentric from my neighbours cutout into mine - until the road guys came in to sort out the joint under the pavement.

They told me it's not at all uncommon.

 

[snowhead]

They told me it's not at all uncommon.

That's what they said at my Son's.
When I say they, that wasn't Western Power the D.N.O it was HASTE, the contractor who repaired or replaced the tens of thousands of pounds worth of appliances in the 50 or so homes.
And did it in a swift and very organised manner.
It's one of their speciallities.

 

[ppelec100]

if the supply neutral fails on TNC_S you will
1.lose your earth (might have some sort of earth through the bonding to gas and water).
2. any neutral cables where there is a load connected will be at 240V.

Soo because of point 2, this is why an RCD/RCBO is used as additional protection?
Does this mean any loads will be at 240v in the MAIN building or the OUTBUILDING or both?

 

[pc1966]

Soo because of point 2, this is why an RCD/RCBO is used as additional protection?

Unfortunately in this case a RCD or RCBO offers no protection for a shock from L to E because the normal supply (L-N) has gone so it won't trip.

But more fundamentally, the CPC has become live from the N-E link before the RCD, so it will not even see a CPC to "true Earth" leak!

Does this mean any loads will be at 240v in the MAIN building or the OUTBUILDING or both?

In the PME fault case both will be at the elevated voltage.

If the out building was on TT then at least the CPC/metalwork there is still at 0V. But with the loss of L-N volts you don't have any RCD protection from further faults.

 

[ppelec100]

In the ideal TN-S case, yes. A fault on neutral has no impact on the E connections.

However, in reality most DNO networks are not "pure" with only an ideal case, so what looks to be TN-S at the home cut-out could actually be TN-C at some point between there and the transformer (where ideally the N-E common link is located).

In most cases the best route is to connect the earth/CPC together, as most out buildings won't have extraneous conductive parts to worry about and most will not present a risk even if there is a PME fault.

The armour would then be glanded at both ends for a good connection. But you might still need to use 3-core for a copper CPC, or to have an external CPC run along side the SWA (might be cheaper, not a good approach unless in duct), for the following reasons:

• If PME and extraneous, to meet the 10mm copper equivalent minimum
• To meet the Zs for sub-main and/or final circuits disconnection times

You need to get to around 70/50/35mm for 2/3/4-core for the SWA armour to be "10mm copper" equivalent, and for even moderate lengths of typical sizes (like 4-10mm for modest loads/medium runs) you can find the sub-main R2 is too high for acceptable disconnection on the armour alone.


The main home also has the PME risk, but it should be adequately bonded for water/gas pipes, etc. With a 60-100A main supply and incoming cables of 16-25mm you would not really look at less than 16mm for main earth and 10mm for main bonds anyway.

The PME risk is really about voltage differences. Inside a home with adequate bonding you don't see the "true Earth" to get a shock. Outside where you have access to the CPC and the Earth you have that problem.

So it is not that the main home is intrinsically safe and an outbuilding unsafe, but more likely an outbuilding will be used for outdoor stuff, hence the greater concerns.

Okay I'm kind of on board here. Soo because you are most likely to be exposed to the earth/ground itself and you will become the earth fault path if a fault does occur (if the tn-c-s supply system fails) this is why they ask not to connect the main buildings supply cable armouring/cpc to the outbuildings earthing terminal? But even if you do, I mean its still providing protection whilst the tn-c-s system is still operating? Rather than you becoming the fault path to earth... correct me if I'm wrong.

So lets say for instance if you supplied a shed from the MAIN building TN-C-S system or TN-S supply, and then the shed supplied some outdoor light fittings like a lamppost which is made of metal and some ground spike lights which are made of metal too which are both earthed from the fuseboard of the outbuilding.
Q. Would you earth the armouring and cpc at both sides (main AND shed) of the fuseboard. Also would a TT earth rod system need to be installed from/to the sheds fuseboard in case of a TN-C-S or "TN-S" neutral/earth failure? Or is the RCD as an additional protection sufficient enough in case of the failure?
Q. If so... I don't get why a house is any different, I mean you mentioned the earth and gas pipe being bonded which would also bond to the consumer unit through an MET so therefore the accessories would be earthed? Is this right? What if the house had an incoming plastic water pipe and no gas?

Sorry I am asking so many questions I am just trying to get my head around this and I feel like it is a job I will come across a lot in the future and want to do things correctly and safely. If there are any regulations which you could point me to, that would be greats also.

Thanks

 

[Wilko]

page 80 5.5.2 (e) mentions cables should need to withstand a temperature of 170 degrees C.

I’ve an older edition of IET Electricians Guide to the Building Regulations so apologies if I’m off song here. When I look at 5.5.2 it’s part of hot air saunas and cables in Zone 3 must be suitable for 125C ambient and withstand up to 170C. This will require specialist silicon insulated cables, not PVC.

page 82 5.7.3 supplies to outbuildings mentions if an armoured cables supplies an outbuilding such as a shed or garage, care must be taken to only connected the armouring is not connected to the outbuilding terminals, it must be connected to the MAIN buildings earthing terminal.

This is from the section dealing with TN in the house and TT in the shed. If the shed is TT then the armour and the distribution circuit CPC must not be utilised in the shed. They must be connected at the house end so the cable and distribution circuit is protected, but then insulated at the shed.

 

[ppelec100]

Unfortunately in this case a RCD or RCBO offers no protection for a shock from L to E because the normal supply (L-N) has gone so it won't trip.

But more fundamentally, the CPC has become live from the N-E link before the RCD, so it will not even see a CPC to "true Earth" leak!

In the PME fault case both will be at the elevated voltage.

If the out building was on TT then at least the CPC/metalwork there is still at 0V. But with the loss of L-N volts you don't have any RCD protection from further faults.

So theoretically speaking, hypothetically: if there was an incoming water pipe which was INSULATED and no gas pipe, and a PME/TN-C-S failed the metal accessories could become live and will not trip any protective device as there will be no active earthing system in place for the house?

Now if you did TT the shed, then the sheds stuff (any tubs metal light fittings etc) will be earthed, BUT because the regs have said NOT to earth the armouring to the OUTBUILDING you are then lacking earthing in the MAIN building (house)... So shouldn't a TT earth ground rod be installed for the house too then or shouldn't they advise to connect the armouring/cpc to the outbuilding too as well as have a TT ground rod for the outbuilding?

Sorry for the questions just trying to get my head around this. Thanks also for answering.

 

[ppelec100]

I’ve an older edition of IET Electricians Guide to the Building Regulations so apologies if I’m off song here. When I look at 5.5.2 it’s part of hot air saunas and cables in Zone 3 must be suitable for 125C ambient and withstand up to 170C. This will require specialist silicon insulated cables, not PVC.

This is from the section dealing with TN in the house and TT in the shed. If the shed is TT then the armour and the distribution circuit CPC must not be utilised in the shed. They must be connected at the house end so the cable and distribution circuit is protected, but then insulated at the shed.

Okay I'll research the silicone cable, not that I need it but just for future reference.

For the install then of the outbuilding... how would you insulate the armouring to the outbuildings fuseboard? I'm guessing by using a plastic gland rather than a metal armouring glad? What would be wrong in having the swa glanded at both ends AND a TT system installed in the outbuildings fuseboard?

 

[pc1966]

Okay I'm kind of on board here. Soo because you are most likely to be exposed to the earth/ground itself and you will become the earth fault path if a fault does occur (if the tn-c-s supply system fails) this is why they ask not to connect the main buildings supply cable armouring/cpc to the outbuildings earthing terminal? But even if you do, I mean its

Yes, same reason they don't normally allow TN-C-S on building sites, etc.

still providing protection whilst the tn-c-s system is still operating? Rather than you becoming the fault path to earth... correct me if I'm wrong.

Normally the CPC provides the path to true Earth for any fault current, so usually it is the safer option.

So lets say for instance if you supplied a shed from the MAIN building TN-C-S system or TN-S supply, and then the shed supplied some outdoor light fittings like a lamppost which is made of metal and some ground spike lights which are made of metal too which are both earthed from the fuseboard of the outbuilding.
Q. Would you earth the armouring and cpc at both sides (main AND shed) of the fuseboard.

If you have metal outdoor 'furniture' you need to take care with TN-C-S, though in some cases they also act as good earth rods anyway. In that case the region immediately around it is often not such a risk as the earth rod effect both pulls down the CPC in a PME fault case, but also it pulls up the earth immediately around it.

However, to be effective you normally need quite a deep rod (e.g. 2.4m sort of thing) or a buried mesh.

Also would a TT earth rod system need to be installed from/to the sheds fuseboard in case of a TN-C-S or "TN-S" neutral/earth failure? Or is the RCD as an additional protection sufficient enough in case of the failure?

The RCD is a separate question from the earthing. You can use additional earth rods/mesh to bring down the PME risk, but often it is simpler. easier, and cheaper to go down the TT route.

Then you need the RCD as you are very unlikely to achieve a low enough Rs for the rod to disconnect on OCPD.

Q. If so... I don't get why a house is any different, I mean you mentioned the earth and gas pipe being bonded which would also bond to the consumer unit through an MET so therefore the accessories would be earthed? Is this right? What if the house had an incoming plastic water pipe and no gas?

What matters is all of the things you can simultaneously touch are at a similar enough potential that you don't get a shock. If you have plastic service pipes they can't introduce a potential difference (or more precisely, they are high enough in impedance that they wont allow enough of a fault current to put you at risk).

Sorry I am asking so many questions I am just trying to get my head around this and I feel like it is a job I will come across a lot in the future and want to do things correctly and safely. If there are any regulations which you could point me to, that would be greats also.

Asking, thinking, and asking again is the route to learning!

The IET Guidance Note 8 "Earthing & Bonding" is a very useful reference to have, though it is not really a tutorial style of book. But it covers a lot of the questions you have and usefully has in the margins a note of which regulation in the BS7671 is relevant. Sometimes they have suggestions of good practice that are not explicitly in the regulations, for example having continuous conductors (no breaks in bonding) when ever possible so a loose screw won't allow down-stream bond points to become open as well.

The IET's On-Site guide is also a handy book to have, far more compact than the regs and has the most common things in there with earthing and bonding in chapter 4. It also has some really handy tables of standard circuit limits that safe you from doing the calculations in many case. For example, you can look up Table 7.1(i) for a combination of cable size and OCPD and it tells you the maximum length in common cases. If your installation is within these limits you don't have to calculate as you know it meets Zs, etc.