Standby Generator Earthing Question

[Dave-sparks]

The 20 Ohm limit seems at variance with their own guidance on TT earthing arrangements

Earth Electrode Testing

The NICEIC offer their technical support regarding the necessary earthing arrangement to comply with the requirements of BS 7671. Where an earth electrode provides the means of earthing for an install

professional-electrician.com

Also no mention of the <25V PD guidance for farms etc.

The NIC are good at talking nonsense sometimes.

But also earthing for a TT system has different requirements to earthing for the source of a TN system, which is what a generator earth electrode is doing.

 

[Dave-sparks]

Got to love the NIC
I draw your attention to this contradiction from earlier in the article
First it says to water the area around the rod and then not to as it will invalidate the test!!

A sufficient reduction in the resistance of the temporary electrodes may be achieved by driving longer temporary rods into the ground, or by watering the ground in the immediate vicinity of the temporary rods.

The ground adjacent to the earth electrode under test must not be treated with water or brine, as this would make the resistance measurement invalid for the purpose of verification or reporting.

That's not quite the contradiction it first appears.

They are suggesting to water the temporary electrode, which would be the test spikes used for the duration of the test. But not to water the electrode being tested, which is the installed rod etc

 

[mtt.tr]

I've heard this said many times, but nobody has ever been able to point to a regulation to support this.

I appreciate it may have been stipulated in a previous edition of the regulations, but as far as the current regulations, and the last 2 full editions, are concerned it is a myth.

It's good practice it's typical to most earthing and grounding as it stops losing all bonds if one comes free

 

[mtt.tr]

The 20 Ohm limit seems at variance with their own guidance on TT earthing arrangements

Earth Electrode Testing

The NICEIC offer their technical support regarding the necessary earthing arrangement to comply with the requirements of BS 7671. Where an earth electrode provides the means of earthing for an install

professional-electrician.com

Also no mention of the <25V PD guidance for farms etc.

TT and TNs tncs are not equal different rules apply.

Then tncs has other additional requirements

 

[Dave-sparks]

It's good practice it's typical to most earthing and grounding as it stops losing all bonds if one comes free

Oh yes, I agree that joining conductors at a pipe clamp is not necessarily good.

But a bonding conductor can have joints in it, it does not have to be a continuous, unjoined, conductor.

A good example is if the gas meter is moved the main bonding can be joined and extended to the new location.
Or if someone cuts through a bond by accident it can be repaired rather than completely replaced.

 

[mtt.tr]

Oh yes, I agree that joining conductors at a pipe clamp is not necessarily good.

But a bonding conductor can have joints in it, it does not have to be a continuous, unjoined, conductor.

A good example is if the gas meter is moved the main bonding can be joined and extended to the new location.
Or if someone cuts through a bond by accident it can be repaired rather than completely replaced.

For me the exception would becrimped due to reliability.

I agree no specific reg just accepted good practice

 

[Matrixx]

An easy way to tell if a neutral is earthed is to measure from the earth to neutral, if its zero then it is. in not it could be any value as it is not referenced.

You don't need an earth spike if it is permanently installed you connect the earth to the main earth bar usually with a link to allow testing. assuming there is a neutral earth. virtually all three phase Genny's will have a N/E. I have not had much dealings with tiny portable generators, they could be wired up all sorts of ways.

 

[awemawson]

The problem comes if your power cut is because the supply cable is severed including the earth then unless your genny has a spike you have no earth !
We are PME with overhead supply except for the last 50 foot so the incoming cable could easily be cut by falling trees etc in a storm.

 

[brightspark]

The problem comes if your power cut is because the supply cable is severed including the earth then unless your genny has a spike you have no earth !
We are PME with overhead supply except for the last 50 foot so the incoming cable could easily be cut by falling trees etc in a storm.

the earth rod is only any good if you have rcd protection at the source of supply

 

[awemawson]

the earth rod is only any good if you have rcd protection at the source of supply

Which I do

 

[brightspark]

Which I do

thats ok then

 

[aesmith]

When you say "source of the supply", where is the RCD in relation to where you're connecting the generator?

 

[awemawson]

I have inserted a 30mA / 32 amp rcbo in the cable from the portable generator to the house input socket at the generator end. I could have fitted it in the generator itself but being brand new this would have voided the warranty.

 

[Matrixx]

The problem comes if your power cut is because the supply cable is severed including the earth then unless your genny has a spike you have no earth !
We are PME with overhead supply except for the last 50 foot so the incoming cable could easily be cut by falling trees etc in a storm.

if its connected to the main (common earth to the house) earth , its still earthed, regardless of the mains earth being present or not.

 

[awemawson]

No, the main earth point will potentially 'float' as nothing is giving you a ground reference. When we had metal water supply feeds, and metal gas feeds, if all were bonded to the main earth point it would stay close to true ground. But in the modern era with plastic service pipes this is no longer true.

 

[Dave-sparks]

if its connected to the main (common earth to the house) earth , its still earthed, regardless of the mains earth being present or not.

How will it still be earthed if the incoming earth connection is disconnected?

 

[Matrixx]

If you are running on the generator and the generator neutral earth is connected to the main earth bar you are earthed in reference to the generator, the fault current path will be back to the generator , the fault current will go back to the source which is the generator not the mains.

 

[Dave-sparks]

If you are running on the generator and the generator neutral earth is connected to the main earth bar you are earthed in reference to the generator, the fault current path will be back to the generator , the fault current will go back to the source which is the generator not the mains.

But without a connection to an earth electrode you haven't got a neutral to earth link. Connecting one pole of the generator's output to the case of the generator without an earth electrode does not create a neutral to earth link.

 

[Matrixx]

No. the neutral earth is in the generator, it does not need to actually go to earth. you are thinking of a TT type system, where there are no solid earths.

 

[Dave-sparks]

No. the neutral earth is in the generator, it does not need to actually go to earth. you are thinking of a TT type system, where there are no solid earths.

I am not thinking of a TT system, this is a TNS system.

In a single phase TNS system a neutral is created by connecting one pole of the alternator to an earth electrode.
Without any connection to earth the generator supply will be an IT system.

The first letter in these systems denotes whether or not the supply is connected to an earth electrode at the source.
T = connected to earth (from the French word 'terre')
I = Not connected to earth.

If an earth electrode is not connected to one pole of the output then it will not have a neutral because neither pole will be fixed at a neutral potential by that earth reference.