Need Some Advice

[padawan]

HI all I have read these forums for while now, i have always enjoyed reading them, and found them very informative. Finally i get my chance to post a relevant thread. To start i should probably introduce myself a bit, i am currently training to be an electrician, and I'm working towards my NVQ.

So my boss is letting me design an installation and if all goes well it will help to complete a chunk of my NVQ. I have put a rough plan together so far, and was hoping to run it passed you guys, to see if i am going wrong anywhere, or see if there are any hints or tips i can pick up.

The job is running a new sub-main out to an outbuilding (garage in this case), and installing a distribution board, lighting circuit and ring/radial socket circuit. The supply to the property is a TNS system, 80 A supply, with a double pole isolator fitted.

My plan is run 3 core 6mm SWA for the garage supply, from the house consumer unit on a 40a MCB. In the garage install 40a DP RCD, 6a MCB for Lighting circuit, and either 32a/,20a/16a for socket circuit depending on ring or radial. I will run the socket circuit in PVC trunking, and the lighting circuit in PVC conduit. As the SWA will have to be run underground to the garage approximately 5 meters i will use corrugated plastic ducting. On the house side of the SWA i might have to use an adaptable box fixed the external wall, and run 6mm T&E to the consumer unit, as will there be a few 90 degree bends needed which could be problematic with the SWA.

I have a ew questions.

1. As the system is TNS supply, it is fine to export earth to garage supply?

2. On the house side of the SWA am i right that i should i use an MCB so that there is no RCD protection, reducing nuisance tripping? Or should i use a RCBO to make sure the SWA in also protected?

3. Is is best to use 3 core SWA and use 3rd core for earth?

4. I know SWA needs to be glanded both ends, but do both ends need to be bonded to earth? or just the house side?

5. Extraneous conductive parts. Are steel roof trusses an extraneous conductive part? The garage is concrete walls, on a concrete slab. So the trusses are structural steel, but not in contact with the ground, I'm not exactly sure where they stand as an extraneous conductive part.

6. If the trusses are extraneous conductive parts, then am i right that i should bond them in 10mm to the small garage consumer unit, or does it have to be run back to the main consumer unit in the house?

7. Is it best to run a ring or radial socket circuit in the garage? I would prefer 16a radial, but was thinking i might have to go 20a or 32a ring main, as the property owner will want to run a couple of freezers and washing machine etc, which could be an issue with max demand?

8. Does my plan sound ok, or would you suggest a different route? maybe henley block or something?

If you guys think i am on the right track then i will get back to you tomorrow about cable calcs etc. At the moment I'm thinking 6mm SWA, 10mm bonding, 2.5mm2 T&E for power circuit, and 1.5mm2 for lights (possibly singles as run in conduit)

Thanks for reading, sorry my first post was so long lol

 

[Essex1]

Hi.

Great post with good info.

Out of interest, how would you answer your own questions?  I am not trying to catch you out but it will be interesting to have a debate rather than just give over our thoughts.  

 

[padawan]

Hi, thanks for your reply, Im happy for a debate its my favourite way to learn lol.

OK here is my thoughts.

1. As the system in TNS and not PME, i think its best to export the earth. I suppose its just a matter of finding the best way to do this.

2. As the SWA is mechanically protected, with both the armour and ducting, i thought there was no need for RCD protection and it would also reduce the risk of any nuisance tripping. But with 17th edition i know its quite common practice to RCD everything.

3. I thought 6mm 3core would be best, as the 3rd core (CPC) can be used for earth, more reliable than the armour, and saves having to run an extra earth cable. Im not sure if the 3rd core (CPC) is 6mm though as have had limited dealings with SWA.

4. I thought just the house end would need to be bonded as to reduce the risk of introducing any difference in potential etc. Im not sure if it would be safer to bond the garage end also, or if it makes the installation less safe as it were.

5.. I think as the roof trusses are not in contact with the ground, there are exempt as extraneous conductive parts. Although obviously the lighting circuit will be in close proximity to them, and being a concrete garage there could be some reinforcement in the concrete which could poiisibly connect the trusses to ground. I suppose the best way to determine it would be to test, not sure exactly how i would do that though.

6. I think 10mm for bonding, as we normally use 10mm, but haven't done much work with extraneous conductive parts as yet. For ease of running the bonding cable it would be easier to terminate at the garage CU, but i have got this nagging thought that the bonding for extraneous conductive parts needs to be run back to the supply origin.

7. Well due to the circuit load and the fact that any tripping in the garage wouldn't be picked up at the house side, potentially leaving freezer's to defrost for hours unnoticed that 32a ring would cover it. But then again 16a/20a radial should also be ok for a freezer, washing machine and possible portable equipment, which obviously would mean running less cable and save on cost. Not sure which is best route here.

8. Well henley blocks would be best for a submarine i guess, but as there is space in the house CU it seems like the most cost effective method is to tap into the CU. Running the SWA all the way back to the CU and not using a adaptable box would be my preferred method, but i think there will be real issues bending the cable as would need 3 or 4 90degree bends.

Whats your thoughts?

Hi, thanks for your reply, Im happy for a debate its my favourite way to learn lol.

OK here is my thoughts.

1. As the system in TNS and not PME, i think its best to export the earth. I suppose its just a matter of finding the best way to do this.

2. As the SWA is mechanically protected, with both the armour and ducting, i thought there was no need for RCD protection and it would also reduce the risk of any nuisance tripping. But with 17th edition i know its quite common practice to RCD everything.

3. I thought 6mm 3core would be best, as the 3rd core (CPC) can be used for earth, more reliable than the armour, and saves having to run an extra earth cable. Im not sure if the 3rd core (CPC) is 6mm though as have had limited dealings with SWA.

4. I thought just the house end would need to be bonded as to reduce the risk of introducing any difference in potential etc. Im not sure if it would be safer to bond the garage end also, or if it makes the installation less safe as it were.

5.. I think as the roof trusses are not in contact with the ground, there are exempt as extraneous conductive parts. Although obviously the lighting circuit will be in close proximity to them, and being a concrete garage there could be some reinforcement in the concrete which could poiisibly connect the trusses to ground. I suppose the best way to determine it would be to test, not sure exactly how i would do that though.

6. I think 10mm for bonding, as we normally use 10mm, but haven't done much work with extraneous conductive parts as yet. For ease of running the bonding cable it would be easier to terminate at the garage CU, but i have got this nagging thought that the bonding for extraneous conductive parts needs to be run back to the supply origin.

7. Well due to the circuit load and the fact that any tripping in the garage wouldn't be picked up at the house side, potentially leaving freezer's to defrost for hours unnoticed that 32a ring would cover it. But then again 16a/20a radial should also be ok for a freezer, washing machine and possible portable equipment, which obviously would mean running less cable and save on cost. Not sure which is best route here.

8. Well henley blocks would be best for a submarine i guess, but as there is space in the house CU it seems like the most cost effective method is to tap into the CU. Running the SWA all the way back to the CU and not using a adaptable box would be my preferred method, but i think there will be real issues bending the cable as would need 3 or 4 90degree bends.

Whats your thoughts?

sorry not sure why my message double posted, and i don't seem to have provisions to edit it.

 

[ProDave]

I would try my best to avoid having an rcd protecting the submain. The outside bit is okay with that, but you have to be careful how and where you route the internal run to the CU to make sure that does not invoke the need for rcd protection.

Why do you feel the need to put SWA in a duct.  What do you think is wrong with burying it directly in the ground?

 

[apprentice87]

Question for you... There is a specific test to see if a part is "extraneous"

1, Define an extraneous part.

2, How would you test is a part was one?? Clue... there is a test meter involved..

john...

 

[padawan]

I would try my best to avoid having an rcd protecting the submain. The outside bit is okay with that, but you have to be careful how and where you route the internal run to the CU to make sure that does not invoke the need for rcd protection.

Why do you feel the need to put SWA in a duct.  What do you think is wrong with burying it directly in the ground?

The internal run will be clipped direct, as it will be 6mm T&E i could even run it in PVC trunking providing mechanical protection, so that should remove the need for RCD protection?

i was thinking ducting for several reasons.

1. Reduces the chances of damage to the SWA.

2. Saves having to put blinding around the cable.

3. If the SWA ever needs to be moved/removed it would make pulling the cable a lot easier.

4. Last garage sub mains i was involved with we ran ducting underground (by involved i mean i dug the trench lol)

5. Covers the 3rd enclosure for my NVQ.

Or do you think i am over engineering it?

Question for you... There is a specific test to see if a part is "extraneous"

1, Define an extraneous part.

2, How would you test is a part was one?? Clue... there is a test meter involved..

john...

1. The earthed point of the source or an artificial neutral. Extraneous-conductive-part: A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation (4).

​2. Insulation resistance test between the MET and the possible conductive part. Apply 500v and if get a reading of over 23k ohms then its not an exposed conductive part.

I will owe up though, i originally thought it would be a continuity test, but did a little search on this forum for that info. Im guessing would have to use a fly lead for this.

 

[Rob_the_rich]

The Force is strong with this one.

I would have the freezer(s) on its(their) own 16a (or less?) circuit on the non-rcd protected side of a split load garage CU. Or better, its own rcbo (sod the expense eh? all rcbo CU with a main switch). To avoid anything else (eg washing machine) tripping the CU rcd and ruining the food. 

Also, a 32A mcb would probably not discriminate with a 40A one upstream, so radials might be best.

What lengths of cable are we talking of here, of swa and t&e. Thinking volt drop, you might want to do the calcs. Also check calcs for the cable sizes.

Always bond both ends of swa armour, even if using a core for cpc.

Remember that the diameter of the cpc of 6mm t&e may be less than that required for any bonding conductor. So you might want to run another earth cable alongside the t&e. Do the adiabatic calculation too.

Think about the depth of the buried cable and marker tape.

I like your reasons for swa conduit, but in your last point, you already have 3 types of enclosure.

 

[kerching]

Probably one of the best first posts i have seen in a long time....

And you are asking the right questions

And coming up,with your own solutions

9/10 from me so far

Just saying

 

[apprentice87]

There we are!!! you now know how to decide if the roof trusses are extraneous or not!!!

In a situation where a person might be dripping wet or wearing damp clothes in might be better to increase the resistance required to nearer 50k ohms..

I agree, best first post I have ever seen, You will go far i think...

john...

 

[kerching]

There we are!!! you now know how to decide if the roof trusses are extraneous or not!!!

In a situation where a person might be dripping wet or wearing damp clothes in might be better to increase the resistance required to nearer 50k ohms..

I agree, best first post I have ever seen, You will go far i think...

john...

However i think he may not make it as a Spark.......

Most of the ones I have encountered in the Wholesalers are thicker than a Whale Sandwich!....he is asking far too,sensible questions

Just saying

 

[roys]

Yip good first post.

I like putting SWA to an outbuilding in a duct as well, I always include a draw wire in with it as well, proved handy quite recently as I pulled in a 2 core flex which I used for link wires for a house to garage/ workshop intercom. Intercom was 9v but flex was mains rated.

Will you be fitting light fittings onto the metal trusses?

 

[Essex1]

Hi, thanks for your reply, Im happy for a debate its my favourite way to learn lol.

OK here is my thoughts.

1. As the system in TNS and not PME, i think its best to export the earth. I suppose its just a matter of finding the best way to do this.

2. As the SWA is mechanically protected, with both the armour and ducting, i thought there was no need for RCD protection and it would also reduce the risk of any nuisance tripping. But with 17th edition i know its quite common practice to RCD everything.

3. I thought 6mm 3core would be best, as the 3rd core (CPC) can be used for earth, more reliable than the armour, and saves having to run an extra earth cable. Im not sure if the 3rd core (CPC) is 6mm though as have had limited dealings with SWA.

4. I thought just the house end would need to be bonded as to reduce the risk of introducing any difference in potential etc. Im not sure if it would be safer to bond the garage end also, or if it makes the installation less safe as it were.

5.. I think as the roof trusses are not in contact with the ground, there are exempt as extraneous conductive parts. Although obviously the lighting circuit will be in close proximity to them, and being a concrete garage there could be some reinforcement in the concrete which could poiisibly connect the trusses to ground. I suppose the best way to determine it would be to test, not sure exactly how i would do that though.

6. I think 10mm for bonding, as we normally use 10mm, but haven't done much work with extraneous conductive parts as yet. For ease of running the bonding cable it would be easier to terminate at the garage CU, but i have got this nagging thought that the bonding for extraneous conductive parts needs to be run back to the supply origin.

7. Well due to the circuit load and the fact that any tripping in the garage wouldn't be picked up at the house side, potentially leaving freezer's to defrost for hours unnoticed that 32a ring would cover it. But then again 16a/20a radial should also be ok for a freezer, washing machine and possible portable equipment, which obviously would mean running less cable and save on cost. Not sure which is best route here.

8. Well henley blocks would be best for a submarine i guess, but as there is space in the house CU it seems like the most cost effective method is to tap into the CU. Running the SWA all the way back to the CU and not using a adaptable box would be my preferred method, but i think there will be real issues bending the cable as would need 3 or 4 90degree bends.

Whats your thoughts?

sorry not sure why my message double posted, and i don't seem to have provisions to edit it.

1. Agreed.

2. Agreed. Really like the fact you have considered whether RCD protection is actually required here instead of just installing one for the sake of it.

3. Yes all 3 cores will be of equal size. You would be surprised to know that the SWA will probably give you a better quality earth. However you then have issues with terminating properly as all cores should enter a metal enclosure together. Although this is very often not done with SWA and separate earths.

4. Good practice would be to earth both ends. Would I code it on an EICR if only one end was earthed? - no

5. You need to test to be certain. I would guess at no however.

6. Where will you take the bond back to?

7. Never seen a fridge or freezer trip an RCD so I consider this a myth. Speak to the customer and gauge what they require.

8. Henley block is good also but you may need additional protection at the house end.

 

[Andy™]

4. Good practice would be to earth both ends. Would I code it on an EICR if only one end was earthed? - no

i would, but only if it was the load end that was earthed and not the supply

 

[padawan]

Thanks for your replies guys, you have been very helpful

The Force is strong with this one.

I would have the freezer(s) on its(their) own 16a (or less?) circuit on the non-rcd protected side of a split load garage CU. Or better, its own rcbo (sod the expense eh? all rcbo CU with a main switch). To avoid anything else (eg washing machine) tripping the CU rcd and ruining the food. 

Also, a 32A mcb would probably not discriminate with a 40A one upstream, so radials might be best.

What lengths of cable are we talking of here, of swa and t&e. Thinking volt drop, you might want to do the calcs. Also check calcs for the cable sizes.

Always bond both ends of swa armour, even if using a core for cpc.

Remember that the diameter of the cpc of 6mm t&e may be less than that required for any bonding conductor. So you might want to run another earth cable alongside the t&e. Do the adiabatic calculation too.

Think about the depth of the buried cable and marker tape.

I like your reasons for swa conduit, but in your last point, you already have 3 types of enclosure.

As its a garage I'm not sure i would want to have any of the sockets without RCD protection, they could potentially move the freezer or want to use portable equipment at some point?

I didnt realise a 32a would not discriminate with a 40a, i thought the RCD protection at garage CU and no RCD protection at house CU cured the discrimination issue. Could you explain why it won't discriminate? i would like to understand the fault for future reference lol. 

I wiil do all the cable calcs, i will be measuring up the job tomorrow then will be able to them more accurately. I will report back with my findings lol

If i have got to run another earth cable, would i better running 3 x 6mm singles? or running 6mm T&E and a 6mm single? Both methods will have to be run in mechanical protection as no RCD protection for the supply cable.

I was thiking bury the ducting/SWA 450 - 500mm down, then backfill, then tape, then concrete.

Sorry for the NVQ unit  i shouldn't of said enclosure, i meant 3 types of mechanical protection.

Yip good first post.

I like putting SWA to an outbuilding in a duct as well, I always include a draw wire in with it as well, proved handy quite recently as I pulled in a 2 core flex which I used for link wires for a house to garage/ workshop intercom. Intercom was 9v but flex was mains rated.

Will you be fitting light fittings onto the metal trusses?

Well the roff trusses are very thin and would be hard to run anything with them , and the roof is made of concrete sheets so i can't really see any viable fixing methods up there. So i thought wall light's was the best way to go. 

1. Agreed.

2. Agreed. Really like the fact you have considered whether RCD protection is actually required here instead of just installing one for the sake of it.

3. Yes all 3 cores will be of equal size. You would be surprised to know that the SWA will probably give you a better quality earth. However you then have issues with terminating properly as all cores should enter a metal enclosure together. Although this is very often not done with SWA and separate earths.

4. Good practice would be to earth both ends. Would I code it on an EICR if only one end was earthed? - no

5. You need to test to be certain. I would guess at no however.

6. Where will you take the bond back to?

7. Never seen a fridge or freezer trip an RCD so I consider this a myth. Speak to the customer and gauge what they require.

8. Henley block is good also but you may need additional protection at the house end.

3. I planned to terminate into an enclosure butted up against garage CU, then run the 3 cores through to the CU. Waterproof glands both ends, banjo or piranha nut (we've never used piraña nuts, but have seen them come up a lot so want to check them out lol).

I was thinking plastic for this enclosure, as the SWA is 3 core, and since the cable isn't RCD protected i wasn't sure where i stand with metal enclosure. Am i right you could possibly bond the metal enclosure to the garage CU?

4. Coding is a whole other beast to me at the moment.

6. I thought either to the garage CU or back to the MET

8. Such as a switchfuse? If the cable is over 3m in length? Or am i confusing my tails up here. 

 

[binky]

label freezer socket as not being RCD protected, or use RCBO as suggested earlier

 

[Essex1]

Thanks for your replies guys, you have been very helpful

As its a garage I'm not sure i would want to have any of the sockets without RCD protection, they could potentially move the freezer or want to use portable equipment at some point?

I didnt realise a 32a would not discriminate with a 40a, i thought the RCD protection at garage CU and no RCD protection at house CU cured the discrimination issue. Could you explain why it won't discriminate? i would like to understand the fault for future reference lol. 

I wiil do all the cable calcs, i will be measuring up the job tomorrow then will be able to them more accurately. I will report back with my findings lol

If i have got to run another earth cable, would i better running 3 x 6mm singles? or running 6mm T&E and a 6mm single? Both methods will have to be run in mechanical protection as no RCD protection for the supply cable.

I was thiking bury the ducting/SWA 450 - 500mm down, then backfill, then tape, then concrete.

Sorry for the NVQ unit  i shouldn't of said enclosure, i meant 3 types of mechanical protection.

Well the roff trusses are very thin and would be hard to run anything with them , and the roof is made of concrete sheets so i can't really see any viable fixing methods up there. So i thought wall light's was the best way to go. 

3. I planned to terminate into an enclosure butted up against garage CU, then run the 3 cores through to the CU. Waterproof glands both ends, banjo or piranha nut (we've never used piraña nuts, but have seen them come up a lot so want to check them out lol).

I was thinking plastic for this enclosure, as the SWA is 3 core, and since the cable isn't RCD protected i wasn't sure where i stand with metal enclosure. Am i right you could possibly bond the metal enclosure to the garage CU?

4. Coding is a whole other beast to me at the moment.

6. I thought either to the garage CU or back to the MET

8. Such as a switchfuse? If the cable is over 3m in length? Or am i confusing my tails up here.

I agree that all those sockets should be RCD protected. I have never experience a fridge/freezer tripping an RCD so no need to RCD it IMO.

I think a 32A MCB will provide discrimination.

You need to look at the difference between earthing and bonding. They are different.

If you bond a 10mm back to the garage CU which is fed by a 6mm earth do you think that is ok?

I think a SF would be a good idea. You need to protect your cables from overload and the main head fuses may be too large.

 

[Rob_the_rich]

losing a freezer full of food could cost £100s, especially if there are 2 of them as in the first post. So a bit of extra work at the design stage makes sense. 

It is possible that a freezer might trip an rcd, but it is more likely for the washing machine to trip one, so ideally the freezers and the washing machine should not share the same rcd. So rcbos would be a good idea, or at least a split load CU with the freezers on the non rcd protected side.

The freezers need not be plugged into a socket, they could be connected by FCUs, but their position in the outbuilding would have to be decided beforehand. Why would they need to be moved around?

Full discrimination between two devices in series is achieved if only the nearest upstream device to a fault operates. The way I see it is that no two MCBs in series (of whatever rating) will fully discriminate since under high fault currents (eg short circuit between L&N) either could trip, if the fault is at the end of the final circuit (where the appliances are). For overload conditions, there may be partial discrimination between two MCBs in series in that if the current rating of the lower rated mcb is sufficiently lower than the higher one, then it will trip first. Check the manufacturers data for what combination will work.

A better way of achieving full discrimination of two devices in series is to have a fuse upstream (switched fuse off henly blocks) and MCBs downstream (in the garage CU)

 

[padawan]

Hey guys, thanks for your replies again sorry i have taken a while to get back to you but my missus parents came down this weekend so i have been super busy keeping them entertained lol

If you bond a 10mm back to the garage CU which is fed by a 6mm earth do you think that is ok?

 

Ah yeh good point, i didnt think hard enough about that one. So am i right that it would either have to be:

1. 10mm bond run back to the house consumer unit

2. 6mm bond run to the garage consumer unit.

losing a freezer full of food could cost £100s, especially if there are 2 of them as in the first post. So a bit of extra work at the design stage makes sense. 

It is possible that a freezer might trip an rcd, but it is more likely for the washing machine to trip one, so ideally the freezers and the washing machine should not share the same rcd. So rcbos would be a good idea, or at least a split load CU with the freezers on the non rcd protected side.

The freezers need not be plugged into a socket, they could be connected by FCUs, but their position in the outbuilding would have to be decided beforehand. Why would they need to be moved around?

Full discrimination between two devices in series is achieved if only the nearest upstream device to a fault operates. The way I see it is that no two MCBs in series (of whatever rating) will fully discriminate since under high fault currents (eg short circuit between L&N) either could trip, if the fault is at the end of the final circuit (where the appliances are). For overload conditions, there may be partial discrimination between two MCBs in series in that if the current rating of the lower rated mcb is sufficiently lower than the higher one, then it will trip first. Check the manufacturers data for what combination will work.

A better way of achieving full discrimination of two devices in series is to have a fuse upstream (switched fuse off henly blocks) and MCBs downstream (in the garage CU)

There is no real reason why they would move the freezer tbh, i just thought on the off chance they did really. Ah i see, makes sense about discrimination, thanks for explaining it really helped.

Since reading you guys recent replies i have decided it will probably be to best to split the tails at the supply, instead of running the sub-main from the house consumer unit. So my amended plan is to split the tails with henley blocks, feed the tails for the sub-main into a 63A switch fuse, then run my 6mm sub-main from the switch fuse to the garage. I haven't fitted henley blocks before, so its actually a win win situation really, as i will pick up a new skill.

As i haven't done it before can i just check that my installation method is correct?

1. For line and neutral Install 100A DP connector block, such as this ( as opposed to running L and N to separate SP blocks?)

2. Run 25mm L and N tails from DP isolator to the DP henley block.

3. Run 25mm L and N tails from henley block, to the house consumer unit.

4. Run 25mm L and N tails from henley block to the 63A switch fuse for garage supply. Or would 16mm be better for 63A? i was just thinking about forward planning with 25mm.

5. Connect 16mm earth supply to MET in consumer unit. 

6. Run 16mm earth from MET in house consumer unit to the switch fuse for garage supply. 

7. Or is it better to install an external earth bar for the MET? and if so run 16mm from earth bar to switch fuse for the garage supply.

 

[steptoe]

Do NOT use DP connector blocks, they are the work of the devil!!!! 

 

[Lurch]

I don't see why you wouldn't use a spare way in the CU TBH. The likelihood of the discrimination actually being an issue would be outweighed by the extra cost (labour and materials would soon add up) of the separate switchfuse. Depending on the board you may be able to use an HRC in it so you should still get the discrimination and you have saved a good chunk on the price. I like to see decent quality materials specced for the correct methods but I do not like to see OTT installs as a standard option.

As for freezers on FCU's, not a bad idea but a PITA and I have seen people with a new freezer just chop the old flex off and then run an extension from the other side of the garage to power it so completely negates the point of the FCU. You need to consider the usability of the installation. I would probably stick 2 socket circuits in on separate RCBO's if I thought it would be an issue.