Fuse or MCB

[Pat Tester]

I have looked at a new job, House with tncs plus additional earth rod connected into the earth tag on the service fuse. House out in the sticks so I guess the DNO thought 'belt and braces'
They want cable to connect detached garage, will be using swa and separate earth rod for garage (not exporting the the pme) As the swa has mechanical protection, I do not need to protect it with an RCD ?
Would you use a switch fuse to isolate the supply or a two pole MCB. They would be in an external ip rated box close to the meter box behind a large shrub. The rcd can then go into the c/s within the garage.
Or would you fit an rcbo as isolator and not one within the garage.
I just wanted some feedback from others.
Thanks in advance for the usual quality advice

 

[Spudnik]

As you said really.

Use a spare way in the CU as a sub main (non RCD side), swa to garage with a 2 or more way CU with RCD main switch.

 

[GT1]

defo agree with jason
just use a spare way in exisiting cu as a distribution circuit.

still need RCD protection in final circuit due to likelyhood of outdoor equipment being used and non-skilled people using the sockets etc etc

might be worth exploring the earth rod situation..?
was it defo the DNO that fitted it or has the PME been put in afterwards and the earth rod just not removed..?

perhaps the Ze with the PME alone is fine, and fully exportable to the garage..? (depending on distance)
just a thought...

 

[Pat Tester]

Problem is there are no spare ways in the c/s and also it is 3m away from outside wall within new refurbished utility room. So without buggering up their nice new walls I have to put Hendly block after meter and run new tails into new external box.

I thought you could not export the pme ? the run is about 20M. Ze 0.34 ohms

The garage will have a bedroom and shower room installed at a later date.

I don't know if the DNO fitted it, customer moved in recentley so have no idea and no paperwork exists !

Is it necessary or sensible to put rcd on swa ? will 2 pole mcb do ? (so much neater than switched fuse)

 

[mrloy99]

I have looked at a new job, House with tncs plus additional earth rod connected into the earth tag on the service fuse. House out in the sticks so I guess the DNO thought 'belt and braces'
They want cable to connect detached garage, will be using swa and separate earth rod for garage (not exporting the the pme) As the swa has mechanical protection, I do not need to protect it with an RCD ?
Would you use a switch fuse to isolate the supply or a two pole MCB. They would be in an external ip rated box close to the meter box behind a large shrub. The rcd can then go into the c/s within the garage.
Or would you fit an rcbo as isolator and not one within the garage.
I just wanted some feedback from others.
Thanks in advance for the usual quality advice

Just a thought...It might be worthwhile disconnecting(with your mainswitch off)the earth rod cable and testing to see if there is indeed a suppliers earth or if you are relying on the earth rod.I recon there should be a separate isco/henly block fitted with the usual water,gas ,cable to cons unit AND cable to your"service fuse earth tag"
I'm guessing that your external ip box is needed because there are no spare ways or room for any additional switchgear---I recon this might lead to some confusion in the future and it might be worth their while to make a bigger box or install a new cons unit to have all control gear within the same enclosure. OR maybe your installation has already an external ip meterbox enclosure and you are intending to Tap in there in which case a main switch and sp mcb should suffice,providing that the installation IS indeed tncs

 

[GT1]

you can export the pme so long as the outbuilding has no extraneous conductive parts.. ie metal structure.
If your considering so you will have to make sure you can meet the disconection times with the CSA of cable your intending.
Agree that splitting tails with a henley block into a switch fuse in your external box sounds a viable solution.
But i would defo do what mrloy99 advises and measure Ze with the rod disconnected and see how the figures compare. It may even be the other way round, and your not getting much return path at all from the rod..?
May also be with checking with DNO to see if they have any quirky local rules about outbuildings..?

 

[Spudnik]

Remember, if you plan on using a henly block, and the tails to the garage are 3-4m, they will need protection with a switchfuse.

 

[Pat Tester]

Ze 0.34 is indeed the pme earth without the rod connected.
Client don't want damage to utility so only option is external box, not an elegant solution I give you, but
customer knows best !

Thanks for help

 

[GT1]

ahhh... perhaps it is a conversion at some point to PME then, and the rod was just left in situ, as you say "belt and braces" but in reality, contributing c**k all to Ze..!

Be interested to hear what you do with regard to exporting PME or not,, got similar job coming up..
when the weather warms up a bit though..!!!

 

[Pat Tester]

Well if it's permissible to export it then why bother with an earth rod.

Is it ok to use an mcb as the isolator ? or is there some regulation against this ?
if not then why not use a two pole mcb rather than a switch and single pole mcb ?

 

[GT1]

worth searching on here, and generally on the internet about exporting PME..
if its steel structured building then you cant anyway..
and likewise if you cant achieve disconnection times due to length of cable run/CSA of SWA etc
plenty of postings and items available

personaly, based on what you said, i would make sure the calculated Ze your going to achieve is well within limits, due to potential of a shower instal in the future. But assuming your are rating the cable for that too, you should have plenty of CSA available.?
The only possible spoiler is the length of the cable run, which may increase the Ze at the garage CU.
this link briefly explains that single pole MCBs are okay for distribution circuits
Subcategory - 1. BS7671 - Voltimum UK - Electrical Installation Products and Contracting

If the length of the tails from Henley Block necessitate a switched fuse, then I would run from the switched fuse, into a small CU in the garage with RCD main switch. Just ensure discrimination between main DNO incoming fuse, your switched fuse, and the main switch (RCD) at the new CU in the garage..

Im not a massive fan of TT'ing stuff just for my convenience..
its okay on a farmers shed or a domestic garage used for a lawn mower once a week. But local conditions in the ground can change. Whilst the readings you get are okay on the day, will they be okay in the middle of a dry summer..? are you going to go back and check..?

With it being potentialy a bathroom and bedroom in the future, personaly, if i could achieve a low enough Ze and export the PME I would be happier,(as long as the building is suitable) knowing the instalation is pretty much maintenance free and not susceptible to local changes in the ground.. or someone cutting through the earth cable to the rod..!!

Lots of variables ....
down to what you see on the ground at the end of the day...

 

[Spudnik]

DP isolation is only required on TT systems.

And this is achieved with the CU main switch.

 

[mrloy99]

Well if it's permissible to export it then why bother with an earth rod.

Is it ok to use an mcb as the isolator ? or is there some regulation against this ?
if not then why not use a two pole mcb rather than a switch and single pole mcb ?

Hi pat
The craic is if you want to export your earth you would also have to run in your 10mm earth(s)to your waterpipes/gas/oil/structural steel in the garage back to your MET so it is usually less bother to keep the new installation on its own and have a separate rcd+MET and associated bonding in the garage.
As for your DP mcb idea,as Jason says there is no requirement for DP mcbs BUT then again its probably just as cheap as one mcb plus mainswitch and they would both fit into a 2module enclosure.(though I've never seen a dp mcb personally)
Also dont forget that you will still need to earth the swa at the supply end without any chance of it being touched at the garage end at the same time as touching an earthed metallic part,which can be a bit tricky to achieve.
What size swa are we talking here 16mm with 63amp mcb?and where is your meter currently located?At consumer unit position or outside?I believe 2m is the maximum permitted distance from service fuse to where it needs to be fused down

 

[GT1]

if you want to export your earth you would also have to run in your 10mm earth(s)to your waterpipes/gas/oil/structural steel in the garage back to your MET

do you mean in addition to the swa ?

 

[Pat Tester]

The meter is in external box, my box would be located well within the 2m. I will run 16mm 3 core swa.
would this not remove the requirment for another earth ? can I just not bond the metal water if indeed any of it will be copper (plastic from house into garage) straight to the c/s earth bar. Also there would be a 30mA
RCD on the garage c/s for all circuits. I could terminate the swa into plastic box and bring out 3 cores into c/s
this will be flush mounted into new stud wall, meaning that unless you want to climb along the back of a cupboard you cannot contact the armour.