TT and Earth rods

[PEM]

Just replaced old CU on TT with 7 way Crabtree RCBO board.

Original Ze was 230 on a copper tooth pick. Got Ze down to 6.5 ohms using 3 x 4ft 5/8th rods.

Just interested to know how low you guys have got a Ze down on TT and when you'd stop hammering down to Australia?

Cheers

 

[trev]

3 rods coupled, 0.29
Happy days.
My own home, I think was .9 ish. Was definitely below 1 ohm

 

[GMES]

Wazz on it!
Always worked for me, the old school on here will have all done it during their careers :crazy:

 

[Fin170]

Are Crabtree RCBOs double pole?

Got 18 Ohms with a single 4ft twig once

 

[northernsparky]

Are Crabtree RCBOs double pole?

you only need double pole isolation i.e main switch; single pole rcbos are fine as fault protection as far as im aware (if thats where you're going with that comment mate)

 

[davesparks]

No you need double pole protection, not just switching, this is part of the reason for the 100mA TD RCD normally fitted as the main switch in TT systems.

In an ideal world you would have double pole protection for each circuit but until double pole RCBOs become more available it ain't going to be viable

 

[yellowvanman]

No you need double pole protection, not just switching, this is part of the reason for the 100mA TD RCD normally fitted as the main switch in TT systems.

Sorry, but you are wrong, no where in 411.5 does it say DP protective devices are required for TT. For TT you only need DP isolation 537.2.1.1

 

[davesparks]

Seriously? Have you seen the damage an unchecked neutral - earth fault can do?

 

[yellowvanman]

Seriously? Have you seen the damage an unchecked neutral - earth fault can do?

If the line is disconnected then what damage can the neutral do? A neutral-earth fault will trip a SP RCBO.

 

[Knobhead]

If the line is disconnected then what damage can the neutral do? A neutral-earth fault will trip a SP RCBO.

It won’t disconnect any circulating currents N→E.

 

[davesparks]

If the line is disconnected then what damage can the neutral do? A neutral-earth fault will trip a SP RCBO.

It won't disconnect the fault though

 

[yellowvanman]

It won't disconnect the fault though

Correct, but at least the fault won't affect other circuits if they are protected by a common DP RCD.

 

[yellowvanman]

It won’t disconnect any circulating currents N→E.

OK, but these currents will be very small (in most cases) as most of the current(s) will take the path of least resistance which in a TT is the neutral.

 

[Knobhead]

Correct, but at least the fault won't affect other circuits if they are protected by a common DP RCD.

Care to enlighten us all on that little gem?

OK, but these currents will be very small (in most cases) as most of the current(s) will take the path of least resistance which in a TT is the neutral.

Is it? Again enlighten us.

 

[yellowvanman]

Happy to be corrected if I'm wrong, never too old to learn!

I'm just dealing with small (eg. domestic) examples.

Care to enlighten us all on that little gem?

Sorry re-read my post, obviously if a DP RCD was protecting multiple circuits then a neutral/earth fault on one circuit would affect the others on that RCD. What I meant to say was that if RCBOs were being used a neutral-earth fault on 1 circuit wouldn't cause the others to normally trip.

Is it? Again enlighten us.

All the energised circuits would have current returning to the neutral bar, if there was a neutral to earth fault on the tripped circuit then with parallel paths back to the source, some current would 'route' through the neutral to earth fault and some would route back through the neutral, my assumption was that because it is TT the return path via the neutral earth fault would have a lot more impedance than the neutral route and so the current would be that much smaller.

Again, happy to be corrected.

 

[davesparks]

Yes the current returning via the earth rod will be proportional to the current returning via the neutral.
But let's say you've got a Ze of 1ohm and a live-neutral loop of 0.5 ohm, the current will split in a 2:1 ratio.
For a N-E fault on a 1.0mm lighting circuit and a load of 60A on the rest of the installation that gives us 20A flowing the rough the 1.0mm!

 

[yellowvanman]

Yes the current returning via the earth rod will be proportional to the current returning via the neutral.
But let's say you've got a Ze of 1ohm and a live-neutral loop of 0.5 ohm, the current will split in a 2:1 ratio.
For a N-E fault on a 1.0mm lighting circuit and a load of 60A on the rest of the installation that gives us 20A flowing the rough the 1.0mm!

Ok, I did say normally and yes you're correct, but I'm guessing your scenario is very rare! Have you experienced this?

 

[HandySparks]

Yes the current returning via the earth rod will be proportional to the current returning via the neutral.
But let's say you've got a Ze of 1ohm and a live-neutral loop of 0.5 ohm, the current will split in a 2:1 ratio.
For a N-E fault on a 1.0mm lighting circuit and a load of 60A on the rest of the installation that gives us 20A flowing the rough the 1.0mm!

Hmm. Doesn't this mean that a neutral-earth fault on a typical TN-S or TN-C-S supply would have even higher current flowing through the 1mm² lighting cable? This isn't a fault particular to TT.

 

[davesparks]

Bingo, you've got it in one, the big flaw with fitting single pole RCBOs. And the reason a lot of them stipulate that they are not to be used on a TNCS system in the manufacturers instructions.

It is not particular to TT systems, but does get more exciting on them because the step potentials that appear on the ground around the earth electrode at the same time could be somewhat of a surprise to someone walking near it!
Also with a TT the extraneous conductive parts are likely to play a much bigger role in your earth return path than in a TN system, how do you like the sound of that current flowing to earth via your gas pipes.

 

[davesparks]

Ok, I did say normally and yes you're correct, but I'm guessing your scenario is very rare! Have you experienced this?

Those figures are completely picked out of the air to illustrate the point.