Right chaps/ladies- this seems to be an appropriate place and thread to get educated better in my understanding. 

Please may someone help me and my understanding of generator earthing requirements. 

I don't do Generators all day every day, but periodically do for construction site set ups and temporary supplies for construction site compounds. This is not my day in and day out job, but not uncommon either. 

So most Generators larger than 10KVa arrive as standard, here in the UK as a TNS system. I believe that is standard operating procedure around here.

So far all mine have. Hired from Large national hire companies.

So the generator runs and operates as a TNS System. Earth fault path is a TNS system. 

However, I understand if the Generator is NOT earth staked, the neutral and earth systems, because they are common at the Genset - can float up and down with reference to terra firma - the ground next to the Generator - and RCDs don't know when a fault is occurring as this floating value confuses the RCDs......, yes?

All of my generators have come with a earth fault relay RCDs that I'd normally set up, (I commonly call these "viggi units") as described in the above sort of way, with a 300mA 0.25 second time delay and cascading RCD settings out from there.

Clearly, this needs to have an earth reference point to tell the RCD what value earth is. Of course it does - no dispute there. 

So - I think I'm all on track so far......

Now, to the earth reference point; the earth stake, matt, or commonly for me, on railway sidings or construction sites out in the country side where a metal fence has been installed, a metal fence that is "owned by" and managed by us, and I have permission to use - then I'd use the metal fence, I like a metal fence................... and its required value in ohms.

So, hypothetically,  lets pretend that the generator was laid on a glass slab, or floating in the air - no contact with the ground at all, and the earth reference point (The earth stake hammered into the ground and connected to the Generator earth point for example) is effectively an infinite value as an earth fault path back to the Generator, but it does provide the earth part of the generator with a reference point to what the earth value should be. I understand that the earth fault relay RCD "viggi" unit would function perfectly well.

So to my question - why should we try to achieve a 20 ohm value for this earth reference point - its just an earth reference and not an earth fault path - surely? 

(Obviously I'm not talking about a TT system here - I understand the need for that earth stake to be less than 200 ohms and all its difficulties, I think)

If the generator earth reference point was 200 ohms or 2000 ohms, would it make a difference? I don't think so.....

I feel an ohms Law coming on as the explanation here.........(V=IR maybe?)

But it's not a fault path, just a reference point........Education required please.

If i stuck a good old metal shaft screw driver in the ground it'd (The RCD part of the Genny) probably function perfectly well surely?

Your understanding is sound, but there are a couple of extra situations to consider,

1)

Actually even without the earth rod there is merit in the RCD for 'glass slab' systems, or vehicles on tyres as we more commonly find the nearest approximation to that - the CPC may bond solidly to the chassis but a human in between an exposed  live and the vehicle body at potential of the system CPC is a few k ohms and will not trip an MCB.

But an RCD sees the current coming back via the CPC as 'missing' from the expected neutral current, and trips on the imbalance between L and N current in the normal way.

2) 20 ohms is a bit arbitrary but it cannot be too high - now our genset is on the ground and the earth rod is in the dry and not a great contact.  Any fault to that connects live to terra-firma (wire pinched on the rigging of a lighting tower, socket box falls into a ditch full of water...) will cause current into the Terra-firma at the fault, and back out of terra firma back to the genset star point at the genset earth electrode. This current raises the genset case and the rig CPC relative to terra-firma, and we do not want the rise to be too painful for anyone leaning against any 'earthed' metalwork connected to the genset CPC loop.

With a 300mA and delay set on the 'Viggy'  unit, 0,2 amps (plus some) could be flowing all day, so a hundred ohms would give a rise of 20V plus and 200 ohms would be 40V or more and some rise. 2000 ohms would never operate that RCD.


A safety factor of ten from a 50V rise with the highest common RCD setting is a very safe starter for ten with a lot of allowance for seasonal variation and corrosion.. but  yes, 20 ohms is to be seen as an aspiration, not an essential.

Mike

tattyinengland said:

If the generator earth reference point was 200 ohms or 2000 ohms, would it make a difference? I don't think so.....

Well, funny you should pick 200 ohms ... Note to Table 41.5 of BS 7671 tells us that above 200 ohms, an earth electrode may not be stable - a physical fact, which rules out 2000 ohms.

Interestingly, since 1st Ed of the IET CoP for Electrical Energy Storage Systems, for "small" battery storage/PV systems (say up to 10 kVA total generation), 200 ohms (the limit of stability) is often seen as the limit. In the 2nd Ed, the 10 kVA "limit" above which 20 ohms max earth electrode resistance is required was removed, and this has remained in the 3rd Ed - all "prosumers installations" should have an earth electrode not exceeding 200 ohms.

So it seems your line of thinking is in line with the experts involved in that Code of Practice. (Note also, though, with the application of prosumer's electrical installations, unless the system is TT, the consumer's earth electrode will usually be in parallel with the distributor's means of earthing, so unless the supply cable is being worked on, more often than not the effective earthing resistance of the installation is lower in any case.)

As you've already identified, there is an exception for TT systems if there are any TT circuits supplied from the system, the total loop impedance (based on the RCD residual current rating) should not exceed the values in Table 41.5 of BS 7671.

So, yes, you are correct, a value of greater than 20 ohms may be acceptable, but the value ought not to exceed 200 ohms.

BSI have announced a project to revise BS 7430, which is under way, so it will be interesting to see if there is any movement on the basic "20 ohms" that is currently recommended.

tattyinengland said:

If the generator earth reference point was 200 ohms or 2000 ohms, would it make a difference? I don't think so.....

Well, funny you should pick 200 ohms ... Note to Table 41.5 of BS 7671 tells us that above 200 ohms, an earth electrode may not be stable - a physical fact, which rules out 2000 ohms.

Interestingly, since 1st Ed of the IET CoP for Electrical Energy Storage Systems, for "small" battery storage/PV systems (say up to 10 kVA total generation), 200 ohms (the limit of stability) is often seen as the limit. In the 2nd Ed, the 10 kVA "limit" above which 20 ohms max earth electrode resistance is required was removed, and this has remained in the 3rd Ed - all "prosumers installations" should have an earth electrode not exceeding 200 ohms.

So it seems your line of thinking is in line with the experts involved in that Code of Practice. (Note also, though, with the application of prosumer's electrical installations, unless the system is TT, the consumer's earth electrode will usually be in parallel with the distributor's means of earthing, so unless the supply cable is being worked on, more often than not the effective earthing resistance of the installation is lower in any case.)

As you've already identified, there is an exception for TT systems if there are any TT circuits supplied from the system, the total loop impedance (based on the RCD residual current rating) should not exceed the values in Table 41.5 of BS 7671.

So, yes, you are correct, a value of greater than 20 ohms may be acceptable, but the value ought not to exceed 200 ohms.

BSI have announced a project to revise BS 7430, which is under way, so it will be interesting to see if there is any movement on the basic "20 ohms" that is currently recommended.

I picked 200 ohms as that value keeps coming up in various literature for earth stakes -  I multiplied it by 10 just to get an extreme distance from the 200 ohm value and twice the human bodies apparent value.

Another value that keeps coming up is 20 ohms - this earth stake for Generators and in the rail industry, I have done a lot of metal work bonding - with overhead HV supplies - and the 20 ohms value comes up there too. Thanks for the reply. I'm sure I've got the IET temporary power supplies booklet - bought at the Elex show - I must read it again and pay more attention. I assume it's based on BS7430. 

tattyinengland said:

I assume it's based on BS7430. 

The 20 ohms is from BS 7430.

If we look back in history of electrical standards for earthing, sometimes 10 ohms has been quoted, sometimes 20 ... but never with a given reason.

One of my [esteemed] colleagues looked into this at some length and couldn't find a definitive documented "rule of thumb" or "formula" that either 10 ohms or 20 ohms was based on (although I guess we could all come up with a variety of fault current values vs touch-voltage or touch-current using ohms law)?