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.