Earth rod query

[IQ Electrical]

All this talk about earth rods and I have one query.

When measuring an earth rod resistance and trying to get it down to 10 ohms are we talking about the rod resistance itself or are we talking about the EFLI?

If we are talking about the rod resistance being ten ohms then what about the impedance of the transformer rod which could be twenty ohms itself so no matter how low we get our earth rod resistance we are never going to get lower than the impedance of the transformer rod.

Admittedly there will be parallel paths involved through other peoples bonding etc but when talking about a low Ra are we talking just about the earth rod or are we talking about the total earth fault loop?

Thanks.

In theory, when we use an earth fault loop impedance tester, the instrument measures Ze because it gives us Ra for the earth electrode and earthing conductor, the resistance of the supply transformer electrode, the transformer winding impedance and the resistance of the supply line conductor from the transformer.

 

[drew35]

If you take this argument to the extreme, once you start ignoring one regulation why not just ignore them all? Then you don't need to bother with an earth at all!

 

[IQ Electrical]

If you take this argument to the extreme, once you start ignoring one regulation why not just ignore them all? Then you don't need to bother with an earth at all!

Which regulation was ignored?

 

[Engineer54]

what about the impedance of the transformer rod which could be twenty ohms itself

You would have to search long and hard to find a sub-station earthing system anywhere near that 20 ohm max. Here they do use multiple rod positions and the 2.4/3.2m rods, for both the MV and the LV side of things!!

The best way to test an earth electrode of any type, is with a dedicated 3/4 pole earth test kit. unfortunately most of the even half decent ones are too expensive for the self employed electrician that only comes across TT system from time to time.

Some MFT's have a facility for earth testing, but then your only getting what you pay for, so the facilities are very basic...

​

 

[La Poste]

I was thinking about our local TX.

I wonder what the resistance of the earth electrode is on this:

 

[Knobhead]

For interest here’s the arrangement for a DNO substation.

View attachment 13578

 

[Silly Sausage]

Why is there an 8m min distance on your diagram?




You don't have to answer that question of course
Way out of my instrument league.

 

[La Poste]

The reason for the 8 metres is so the LV Earth does not pick up voltage from an HV Earth fault and transfer it to the LV system.

There is a rule that if an HV Earth is 1 Ohm or lower the LV and HV Earths can be combined but don't ask me why.

An HV Earth is also known as an Earth nest (multiple Rods perhaps) but again don't ask me why but I like the expression an "Earth nest".

 

[trev]

I was thinking about our local TX.

I wonder what the resistance of the earth electrode is on this:

You need to get out a bit more, there are these things called beer, women and things to do.
Sorry mate I couldn't resist

 

[jaresquire]

I often take the kids to the local park and whilst they run around creating mayhem I'm stood there looking at the sub-station in the corner..

 

[IQ Electrical]

I often take the kids to the local park and whilst they run around creating mayhem I'm stood there looking at the sub-station in the corner..

Talk about suspense! What's it like????

 

[malcolmsanford]

I often take the kids to the local park and whilst they run around creating mayhem I'm stood there looking at the sub-station in the corner..

I'm starting to really worry about you.........................

 

[jaresquire]

Big and green with swings and a slide!

 

[malcolmsanford]

The sub station?????

 

[trev]

Big and green with swings and a slide!

Only if Carlsberg built substations

 

[jaresquire]

If the DNO thought it would pay then it might happen

 

[Knobhead]

Why is there an 8m min distance on your diagram?

As La Poste has said to prevent imported MV faults appearing on the consumers system.
The drawing does show a removeable MV / LV link. All the transformers I’ve installed have had the MV to LV solidly linked.

This makes interesting reading http://www.cablejoints.co.uk/upload/Earthing___Guidance_Notes__Central_Networks.pdf

 

[Engineer54]

As La Poste has said to prevent imported MV faults appearing on the consumers system.
The drawing does show a removeable MV / LV link. All the transformers I’ve installed have had the MV to LV solidly linked.

That's because they were all probably well under the 1 ohm mark.... lol!!!

 

[Silly Sausage]

As ......................

I might have a read later. Thanks anyway.
It just seemed a specific value rather than, say, a 'round' 5 or 10m.