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?

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.

lets pretend that the generator was laid on a glass slab, or floating in the air

You don't have to imagine too hard - look at section 717 for mobile/transportable units - we can have exactly that situation.

   - Andy.

AJJewsbury said:

You don't have to imagine too hard - look at section 717 for mobile/transportable units - we can have exactly that situation.

Note though that in this instance, if leads are run outside the unit, and damaged such that a line conductor becomes connected to Earth (the ground - or something metal in good contact with the ground), then the frame/chassis/container of the unit becomes LIVE, and no protective device would operate ... except strategically-placed RCDs but even then only after someone touches the live frame/chassis/container.

Indeed - I have seen exactly that - a shock from the door handles of a genset towing vehicle  parked up & supplying (from memory) radio gear and lights.

really there is a step change between using a genset on a moving vehicle with totally enclosed loads and wiring and the same vehicle parked up and with a lead running out over the grass or electrical loads that are on the ground beside the vehicle.

As soon as that second stage is reached, a connection to terra-firma of at least RCD trippig quality really  should accompany it.
Mike

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. 

Ah - so the value of 200 ohms keeps the touch voltage to 40 odd Volts - R<50V/Ia - has been used I assume?

Less than 200 ohms then ever less touch voltage.....

Now - this is where my uderstanding might be falling over - if that Genny is on wheels, a glass slab or up in the air or something similar and an earth stake is installed - then there wont be a path back to the generator, via the earth stake - clearly there's a fault path via the TNS path.........and perhaps a division of fault current (depending on the resistance) -down to terra firma via the earth stake. - Like a Zs would be infinite - or as near to - if you did a live or dead test - from the Genny and tested from the Genny Live to earth stake (disconnected from the Genny MET)

In old fashion parlance - an ELCB would work. before even my time, but obviously they're still around.

However our modern viggi units also work, (don't they? won't they?) because the earth stake gives the RCD an earth reference to work from, and holds the Genny frame to a local earth potential - that is different to the neutral value - which is also held, now at a local earth value - and allows a current loss too - to somewhere other than the TNS fault path, and the RCD functions....

So in this case the 20 ohms, 200 ohms, or 2000 ohms path back to the genny is irrelevant surely? only a good contact with earth? it'd give an earth reference, its not a fault path.........

I'm still tripping over the same mis understanding.........

Kind Regards

Chris

Thanks AJJewsbury....

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)?

Note though that in this instance, if leads are run outside the unit, and damaged such that a line conductor becomes connected to Earth (the ground - or something metal in good contact with the ground), then the frame/chassis/container of the unit becomes LIVE, and no protective device would operate

You can get a very similar effect with a normal (e.g. DNO) supply too - a downed overhead line conductor in a puddle or just an uncleared L-PE fault in a TT installation - current flows to true Earth and returns through the generator/transformer's electrode - the current * electrode resistance producing a voltage difference between the general mass of the earth and the system's PE reference ... which is then exported to all connected TN installations.

   - Andy.

So in this case the 20 ohms, 200 ohms, or 2000 ohms path back to the genny is irrelevant surely? only a good contact with earth? it'd give an earth reference, its not a fault path.....

It's not a fault path for a conventional ADS style L-PE fault, correct. In an additional protection kind of situation (e.g. picking up the end of a severed flex while stood outdoors) it does though. Likewise for the (rare) situations where a line conductor come into contact with true Earth without an PE conductor intervening. On larger systems you start to see stray capacitance and general leakage currents due to imperfect insulation having similar effects.

    - Andy.

So in this case the 20 ohms, 200 ohms, or 2000 ohms path back to the genny is irrelevant surely? only a good contact with earth? it'd give an earth reference, its not a fault path.....

It's not a fault path for a conventional ADS style L-PE fault, correct. In an additional protection kind of situation (e.g. picking up the end of a severed flex while stood outdoors) it does though. Likewise for the (rare) situations where a line conductor come into contact with true Earth without an PE conductor intervening. On larger systems you start to see stray capacitance and general leakage currents due to imperfect insulation having similar effects.

    - Andy.

Likewise for the (rare) situations where a line conductor come into contact with true Earth without an PE conductor intervening.

Not so rare. That is your expensive Ho7 style  rubber 3 phase 100A extension lead  being thrown over a metal fence and snagging on a sharp edge, and striking a phase core first, or the free socket on the far end going 'splosh' as it lands on the other side .

While in a perfect world this would not occur, it can and does happen when things are being deployed in a hurry and / or  in bad light.

Something like SY or CY cable that actually had a low enough resistance braid to operate a large MCB would help with the snagging, but the flooding has no easy fix. If your genset has sockets for leads for unknown loads to plug into, it needs an electrode, an  NE bond and an RCD.