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

 

[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.

 

[Engineer54]

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

This time next year you could well be looking at an Ra of under 3 ohms, after the surrounding soil has consolidated around the gap made by the extension couplers.

Depth is always going to be the real answer to gaining good stable Ra values. Plenty of other methods, as well as the use of chemical enhancement, but they will all have a cost factor over and above driving suitable size rods into the ground!!

 

[Engineer54]

Are Crabtree RCBOs double pole?

Got 18 Ohms with a single 4ft twig once

More fool you for using a one of those god awful twigs in the first place. One things for sure, it won't ever be a stable Ra value....

 

[Engineer54]

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

.

Do you have a link to a manufactures instructions that states single phase RCBO's are not to be used on PME/TNC-S systems?? I'm just looking at a Schneider single phase RCBO manufacturer's data sheet and it states nothing of the kind. It does recommend not being used for TT systems though!!

 

[davesparks]

Schneider is the one that springs to mind actually, it was their instructions that were used as the example when someone explained all this to me!

 

[Engineer54]

Schneider is the one that springs to mind actually, it was their instructions that were used as the example when someone explained all this to me!

Certainly nothing mentioned on anything from Schneider that i can find. I actually had a quick look at the contractors Schneider technical manuals when i had coffee over there this morning. Only recommendation was better to make use of DP RCBO'S on TT earthed installations....

 

[yellowvanman]

This isn't a fault particular to TT.

Nor is it particular to RCBOs.

What about MCBs on TNCS system, the associated MCB may not even trip on the faulty circuit. At least on a TT (and I'm no fan of TT) something trips and you have to investigate why!

 

[davesparks]

MCBs don't detect neutral - earth faults, they detect over currents and break the fault.

SP RCBOs detect a neutral - earth fault and then break the live! Where is the sense in a protective device which detects a fault and then effectively masks it?

 

[Knobhead]

Does it matter what the regulations or anyone else says. The application of SP RCBO’s is flawed by virtue of basic electrical theory.

We are talking about clearing a fault, not DP isolation.

 

[davesparks]

Can you hear an echo Tony? Or was this track just recorded in stereo?

 

[Knobhead]

The echo takes time in the eather.