TT system scenario - Thoughts please ?

[Lucas123]

Massive site in deep rural kent few big dwellings, swimming pool etc etc

there is an outer building/shed/store (not sure of use) room on the site as well

there has been a seperate feed brought down off the overhead line into this little box built (to house cut outs and meters for all the different supplies) directly below into a single phase cut out to supply this outer building

in the outer building there is a newly installed metal clad db with one lighting circuit and couple of power radials all satisfactory wiring, containment and dead test results but what would be the best method to supply this ? the run is about 30m from the cut oust and meters housing

and with regard to earthing it obviously at the housing there is a earth block fed from a earth electrode but thats as far as that goes

so basically there is main cut out thats it at the minute so would to start with be out the main head to the meter ( being installed friday) out the meter into an 100ma RCCB (db fitted in the outer building has a normal main switch no time delayed rcd)
then out the rccb to the db main switch ?

 

[telectrix]

you post is not very clear. gonna still need a 30mA RCD if only for socket outlets.

 

[malcolmsanford]

I would first ascertain your Ra at the earth electrode. As there is no supply then it the old electrode tester method.

I'm a bit lost about the set up to be Honest, You say that there is a seperate feed into a little box, is this seperate from the house feed ?

 

[Des 56]

You mention time delay rcd
What about overcurrent for the distribution cable to the outbuilding

 

[Chr!s]

Massive site in deep rural kent few big dwellings, swimming pool etc etc

there is an outer building/shed/store (not sure of use) room on the site as well

there has been a seperate feed brought down off the overhead line into this little box built (to house cut outs and meters for all the different supplies) directly below into a single phase cut out to supply this outer building

So all supply's are TT i assume ?

in the outer building there is a newly installed metal clad db with one lighting circuit and couple of power radials all satisfactory wiring, containment and dead test results but what would be the best method to supply this ? the run is about 30m from the cut oust and meters housing

Underground SWA?

and with regard to earthing it obviously at the housing there is a earth block fed from a earth electrode but thats as far as that goes

You can always add an additional electrode at the outbuilding.

so basically there is main cut out thats it at the minute so would to start with be out the main head to the meter ( being installed friday) out the meter into an 100ma RCCB (db fitted in the outer building has a normal main switch no time delayed rcd)

100 mA TD thats fine for earth fault protection, you will also need to provide protection of the live conductors.

then out the rccb to the db main switch ?

You need to consider discrimination/selectivity

Volt drop etc

Will there be any bonding requirements?

Regards Chris

 

[Lucas123]

yeah there is a small brick built enclosure directly under the over head line pole which houses a three phase cut out and two single phase cut outs and their respective meters its hard to ascertain exactly whats doing what as there are 3 rather large dwellings and a smaller anex type building and this outer shed type building which stands alone quite back from the dwellings

i knew when i started writting it would become less clear !!

all TT yes

there has been a new service cable brought down from the overhead line into 100A cut out completely seperate from any other service head

and thats all there is so far to feed this outer building a brand new single phase cut out

so from posts so far
ascertain Ra, if acceptable, would you run a seperate main eartthing conductor from existing earth block which in turn is protected by the existing earth electrode which is doing all other supplies at present (if room permits in block) ?

Then for feed to db
out of rccb after calculating correct size swa into db in outer building with 30ma rcd protecting socket outlets inside this outerbuilding

now meter man is asking for a cert before he installs the meter ? what cert ?

 

[Octopus]

What RCD have you got in the metal enclosure? a 30ma is not permissable IMHO

 

[malcolmsanford]

It's becoming a little clearer now Lucas. Are you doing any work at all on the existing set up , what exactly is your remit in this?

It seems you have this brand new single phase 100amp supply going to a cut out, I'm assuming that is the DNO head. Is this DNO head live?

By the sounds of things this new cutout/head is going to feed a specific outbuilding, is there an existing installation in this out building? And are you going to be adding those sockets etc in this building?

I'll have to jump ahead a little here and guess, from this cutout after the meter is going to be fitted, eventually, are you taking a new SWA feed to a DB, is this an existing DB or a new one you are going to fit?

Now the cert that the meter supply company is looking for is a set of DEAD tests on an EIC, to make sure that the installation he is connecting the meter is safe for energisation. All you do are the dead test, continuity of earthing and CPC conductors, continuity of ring final if you have rings, IR, R1+ R2 and polarity, this is why I was asking you if there is already an existing installation.

 

[Lucas123]

malcolm you are the man

yeah thats it spot on the head/cut out is live

someone has installed a new db in the outer building with one lighting circuit and two radial power circuits of which i have performed all dead tests and are acceptable and the install is very well done in pvc conduit all brand new

my remit is to feed the outer building db from this new cutout

so in essence im not fitting the new db in outer building however it is perfectly installed appears to be absolute brand new and all dead test were of acceptable levels

would you say a straight run from earth block in meter cuboard to outer building db of aabout 30m would suffice ? (correct csa of course)

and yeah eventually a buried swa from meter or rccb to feed new db again obviously 30m run

is there any other things i have to bear in mind when calculating the csa of the swa with it being TT system apart from the obvious

 

[malcolmsanford]

Lucas you will have to supply them a 3 signature EIC with just your signature on the inspection and testing part for those light and radial circuits. The design and Installation side should have been completed by whover done that part, are they available to sign them off?

Jump forward again the meter is fixed and working and you are now going to run your distribution circuit/sub main from the meter to the existing DB that was installed by someone else. First as it is over 30metres you will need to fit a protection device. As the installtion is TT, I would be looking at a 100mA RCD, perhaps even a time delayed one, to protect those tails and a MCB. You could go down a combined RCBO but that is serious money. Whatever you load calcs for the installation is, will be your starting point for the MCB size and Cable CSA.

If you able to I would do an Ra for the existing rod not a Ze, but a proper Ra to get the value. If it was high, and high to me is over 50 ohms then I would be fitting another rod for your installation, you could even tie them together to give a better Ra for the entire installtion. Also another vantage of adding a local rod is, any services or metal that needed bonding in it could be done at the DB and not have to be taken back to the MET. Finally if your rodding local I would be using a 2 core SWA only for the distribution circuit.

Finally is this client using you to get his meter fitted? Why isn't the contractor who fitted the DB and done the lights and Radial doing the ceritification for him? Be careful mate that your not being used here.

 

[AA410]

Hi all i have just installed a 100ma timed delayed RCD on a TT supply to a shed and it will not trip out at 5x on my RCD tester it just says 40ms on the tester any advice please

 

[malcolmsanford]

What meter are you using? any problems with the leads? are you getting a good connection? how many leads are you using?

At 5 x 100mA on S type RCD it should trip min 40mS max 150ms. Often with a time delay you can remove the leads before the device activates.

 

[nickblake]

make sure your testing it on 100ma not the 30 ma setting

 

[gooner1984]

Hi all i have just installed a 100ma timed delayed RCD on a TT supply to a shed and it will not trip out at 5x on my RCD tester it just says 40ms on the tester any advice please

i thought you only do 5x test on 30ma not 100ma

correct me if im wrong

 

[La Poste]

I agree.
you only test a 100 ma S type at 100 ma.
Not at 500 ma.