TT commercial unit upfront RCD

[GBDamo]

Have been doing some small jobs on a commercial unit converted for storage, changing lights and broken sockets and the like.

Units look to be c.20 years old.

The interesting thing is the unit is Three Phase TT, the rod being directly beneath the DB and incomer all in a small cupboard.

There is a 30mA RCD on the tails to the board, no time delay.

The Zs at DB is 0.52Ohms

There are no apparent parallel paths, no gas on site, water plastic and I couldn't see any steel bonding.

It is a very curious set up that doesn't feel right.

Could the Earth Rod be attached to the steel foundations, hence the very low Zs for TT.

With such a low and consistent Ze is this RCD really necessary?

Anyone seen the likes of this, is it common?

 

[James]

You could turn power off, do your ze test with the earth rod disconnected.

I expect the parallel paths may be metal switches or sockets, possibly metal trunking or conduit.

Many of these may be bolted to the steelwork of the building and giving you parallel paths via the cpc’s

 

[GBDamo]

You could turn power off, do your ze test with the earth rod disconnected.

I expect the parallel paths may be metal switches or sockets, possibly metal trunking or conduit.

Many of these may be bolted to the steelwork of the building and giving you parallel paths via the cpc’s

I'm not too worried it was more about the TT up front RCD situation.

 

[01laverickd]

In any case the rcd could be changed to a 100ma s type Rcd which would offer diversity to the downstream circuits.

 

[Strima]

As above I'd look at changing it for 100mA S Type with additional protection for circuits that require it.

Unless you can prove the stability of the electrode, which would probably mean regular testing over the next few months/years to prove it.

I have seen 0.74 ohms on a single rod before but it was quite damp and very near the transformer.

 

[pc1966]

The Zs at DB is 0.52Ohms

To get that with a 3m long 16mm dia rod is going to need a soil resistivity or around 5 ohm.m and according to Wikipedia on soil resistance that is exceptionally low and likely to be corrosive acidic soil (peat bog maybe?)

So as above it is pretty much assured it is not the rod you are reading but some parallel paths within the building to the foundations.

Or even another cable's earth. Has it got any cables to otyher sites? Maybe armoured network cables?

 

[GBDamo]

To get that with a 3m long 16mm dia rod is going to need a soil resistivity or around 5 ohm.m and according to Wikipedia on soil resistance that is exceptionally low and likely to be corrosive acidic soil (peat bog maybe?)

So as above it is pretty much assured it is not the rod you are reading but some parallel paths within the building to the foundations.

Or even another cable's earth. Has it got any cables to otyher sites? Maybe armoured network cables?

Sorry, I've slept since I started this one, remind me what I was dribbling on about?