TT System - Having an RCD Installed

[phykell]

Hi,

My consumer unit is a TT system and I recently asked an electrician to come and check it, and to install an isolator for me as well. This is what I ended up with:

http://homepage.ntlworld.com/phykell/diy/tt.jpg

He said the earth was fine though I have my doubts and I fancy sinking a new copper rod just in case. This is something I can do isn't it?

I have the following questions:

1. Can I get my regional electricity supplier to sort me out with a decent incoming earth converting it from a TT system and if so is this something they're responsible for or me?

2. I really do need an RCD from what I've read on here. Can I just have the isolator fitted with an RCD or is it worth having a whole new CU installed complete with RCD?

3. Will a 100A 30mA RCD be suitable?

Thanks for looking

 

[Steve]

For a start, the tails between the meter and service head/s only look to be 16mm² you could do with these upgrading to 25mm².

The tails from the meter to that isolator are a mix of old and new colours. There should be a label somewhere indicating there are 2 colour schemes in use. The whole thing looks a mess.

That black box and the black wire to the left of the picture - can you pull back the hat on that black cable and post a picture? I presume this is a supply for a shed? Armoured cable shouldn't be bent like that right next to its termination.

EDIT: sorry, didnt realise you already said you have a TT system Did the electrician leave any paperwork saying what the EFLI is, in ohms? If he tested the system, he should have left you a few bits of paper with test results on. And a TT installation MUST be protected with an RCD. a 100mA one is a good choice, and can be fitted in the isolator enclosure.

 

[JohnD]

Not keen on the standard of workmanship:

Grey sheathing cut back too far on meter tails, should enter enclosure

A red and a blue tail! And too long.

Earth wire looks too small

Tails CU to main switch look tight

Don't like that spare red single hanging down

Is the supplier's red tail damaged, or is that a bit of red tape hanging off the red sheathing?

Is that telephone or alarm cable on the right? Should be better segregated or have a barrier if SELV

Can I get my regional electricity supplier to sort me out with a decent incoming earth converting it from a TT system and if so is this something they're responsible for or me?

You can ask the supplier, though they aren't called a REC any more, and the will probably pass your letter on to someone else. You don't have to worry about who it is though, just write to the address on your bill and they will be responsible for knowing. It may or may not be possible, and there might be a charge. You can't do it yourself. I see the supplier's installation is very old, you might tell them that and ask them to confirm that their installation is satisfactory. In my experience of the electricity industry you are much better off sending a concise letter than phoning the call centre.

http://homepage.ntlworld.com/phykell/diy/cu.jpg

 

[SparkyMarky]

The earth cable may actually be ok. I've got a 2.5mm wire to my earth stake at home on a TT system. Wiring regs say that this is ok in some situations on a TT system, ie when the Zs is high enough that the earth fault current won't exceed the capacity of the cable. Of course the only way to confirm this on a specific installation is to test....

 

[Jim2287]

The earth cable may actually be ok. I've got a 2.5mm wire to my earth stake at home on a TT system. Wiring regs say that this is ok in some situations on a TT system, ie when the Zs is high enough that the earth fault current won't exceed the capacity of the cable. Of course the only way to confirm this on a specific installation is to test....

having high Zs is a bad thing isnt it ? why not up the cable size meaning lower touch voltage if a fault occurs.. and faster disconnection times

 

[RF Lighting]

Upping the cable size will make no difference. If the Ze at the earth spike is for example 2.00 ohms no matter what size cable is connected to the spike the Ze will not drop below this value, so if there is a high value of Ze at the spike then this will limit the amount of current that will flow to earth in the event of an earth fault thus permitting the use of a smaller cable.

 

[Jim2287]

When you take Ze do you take the reading when the rod's in the ground ?

 

[RF Lighting]

I have not had a huge wealth of experience on this sort of installation but as far as I understand the Ze value is the reading taken from the earth spike only. (similar to disconnecting bonding etc. when you do a normal Ze test)

 

[james 29]

I think the important thing here is to get an rcd installed within the enclosure which feeds the consumer unit. Regardless of workmanship or cable sizes, no offence intended guys, it would be very unlikely your TT earth is low enough to satisfy the zs requirements of your mcb's.

The size of the rcd could be determined from your ze readings but above 100 ma is recommended. Hope i havent misunderstood your post but if your earthing system genuinely is TT and your electrode resistance is not extremely low, its potentially hazardous and needs urgent remedial work.

The rec may be able to convert and as john mentioned a letter is the best way. They never let you speak to anyone other than a customer service rep with our dno.

 

[Jim2287]

ok supposing you do the test when its in the ground .... what happens if u test on a dry spell .... install small earth cable because Ze is high, then it rains and Ze is lower and too high current could pass for 2.5 cable - ok its not likely to melt the cable, but for the sake of putting 10mm2 in, in the first place


is it ok/a good idea, to install time delay (s type) rcd's on TT ? and then 30ma RCBOs for sockets ?

 

[Spark123]

When you take Ze do you take the reading when the rod's in the ground ?

Yes, the rod needs to be in the ground in contact with earth. If the rod isn't in the ground in contact with earth you will not get a reading.

ok supposing you do the test when its in the ground .... what happens if u test on a dry spell .... install small earth cable because Ze is high, then it rains and Ze is lower and too high current could pass for 2.5 cable - ok its not likely to melt the cable, but for the sake of putting 10mm2 in, in the first place

The cable size is dictated by how it is installed and corrosion, as the system should be RCD protected the main earth conductor will only need to take fault current until the RCD operates.

is it ok/a good idea, to install time delay (s type) rcd's on TT ? and then 30ma RCBOs for sockets ?

Yes

 

[securespark]

Regs say 6mm2 min. on TT, surely?

Also, there must be an S type RCD at least 100mA protecting the whole install, with 30mA on S/O likely to supply equipment for use outdoors.

The TD RCD has to be at least 3x the rated trip current of the 30mA
one(s), ie min 100, but you could have larger, if discrimination will not be achieved using 100TD + 30.

Unless the RCD is on the tails feeding the CU, the CU should be insulated.

As for resistance, I would be happy with <200 Ohms (NIC say 100), but with a 30mA RCD, you can have up to 1666 Ohms, and 500 Ohms on a 100mA RCD.

Some DNO's will convert TT to PME. Ask 'em.

 

[Adam_151]

the what happens if Ze drops and I need a bigger earthing conductor situation is not really an issue for TT, the slowest time anything is going to trip is 0.5 seconds, if you work the adibatic backwards for a 2.5mm conductor you'll find it'll hold 505A for that time (0.45 ohm would be a great Ze on a rod!) (oh and not to mention that if the fault was downstream of the CU*, then even if the breaker was C50 the fault would be cleared inside of 0.1)

*And if an earth fault is able to occur pre-cu on a TT system, you have got problems anyway...

I think the regs about minimum size are more from a POV of making sure it doesn't blow away in the wind

If there were parallel paths through water pipes to a neighbour with TN then higher currents might flow (but not on the cable between rod and MET, just met and CU and met and services) (though if you do the adiabtaic for a BS1361 100A service fuse, you might be supprised at just how small the cables can actually be)

 

[Steve]

And the OP really ought to get a different electrician to do the RCD install. The original electrician did a bad job by just fitting an isolator, he should have fitted an RCD, he left the installation unsafe.

 

[securespark]

Yeah, Crafty, he wasn't a spark. No way would a spark do that.

if you do work to that standard, you're not a spark. He doesn't know enough, clearly.

Name & shame him, that's what I say.