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#138351 - 09/03/03 01:49 PM Generators  
PaulCornwall  Offline
Member
Joined: Jun 2003
Posts: 112
England
Hello Guys

Simple question,,, i would like your thoughts on the issue of generators requiring earth staking.

My thoughts are that it is point less, as the generator supply relies on the center tapping for its earth connection.

For example,, if you loop test the earth rod from the generator supply you wont get a reading,any
earth fault must rely
on the earth supplied from the generator.

a company i some times subcontract to insist on it but personaly i dont see the point..

look forward to your thoughts on the matter


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#138352 - 09/03/03 04:33 PM Re: Generators  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
The problem if you don't ground the generator properly is that an accidental ground placed on the "live" line would go unnoticed, but would leave the neutral at full line voltage.

Moreover, if the neutral and earth terminals on the generator output are bonded together, an accidental ground on the line would also leave all exposed metalwork at line potential.

If you're going to adopt a floating system, then one side of the generator output must not be tied to the frame and to the equipment protective conductors.

To do that properly, you would then need to employ double-pole fusing and switching throughout the rest of the installation.

There would still be no way of detecting a ground-fault on one line unless you installed a PIM.


#138353 - 09/03/03 06:17 PM Re: Generators  
PaulCornwall  Offline
Member
Joined: Jun 2003
Posts: 112
England
Firstly i must point out that the generators i am talking about are for temporary events and not permantly fixed..

I take your point Paul but as you are probably aware 99% of generators have RCD protection, surley an earth fault would take this out anyway,,

As far as i was aware most gennys are earth and neutral bonded at source. Surley that is the case??

The problem is that in theory if you take hold of the live from a genny, and the genny is not grounded you wont get a belt..

What i find hard to understand is that if the earth failed on the genny, and you were left with just an earth rod, (ignoring the RCD for a moment) a live/earth fault occured well nothing would happen, effectively all you would do would be make the earth rod live.

Is this correct??

If thats so i dont see the point of earthing the genny to the general mass of earth,

as you are proberaly aware there are no guide line on this in the regs book..


#138354 - 09/03/03 06:20 PM Re: Generators  
lyledunn  Offline
Member
Joined: Jun 2002
Posts: 159
N.Ireland
Consider a phase to earth fault on a piece of metal equipment connected to a 3 phase floating neutral system. Should a second fault to earth occur on another phase then a phase to phase voltage will appear between the metal parts of the equipment and ground.On the other hand a grounded neutral will reduce that voltage by sqrt 3
Plus it is a requirement of UK wiring Regulations. Small sets serving one single circuit such as you find on construction sites need not be independently earthed as the likelyhood of a double fault situation is low.
Paul has also highlighted dangers which put floating systems beyond the Pale for general use.


regards

lyle dunn

#138355 - 09/04/03 12:43 PM Re: Generators  
PaulCornwall  Offline
Member
Joined: Jun 2003
Posts: 112
England
Hi Lyle

am i missing the point here,, you say ground the Neutral/Earth. but to what??

If you had an earth fault on a piece of metal equipment,( am i to Assume this piece of metal is connected to the genny earth or not) it would blow the trip..


It is grounded at the genny,ie center tapped, i still dont see where your coming from,must be having a blonde day,,

also you say its a requirement in the regs, i cant find it.

As far as i can see a fault to the mass of earth would do nothing..

Help!!!!!!!!!

I am confused


#138356 - 09/04/03 04:54 PM Re: Generators  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
The situation Lyle is desribing is where an otherwise-floating system gets an accidental ground applied to one phase.

Assume you have a 240/415V generator with floating output, and a fault results in an accidental ground on the red phase (maybe a cable trapped in a door which causes a metal frame to cut into the phase wire).

Because the generator output is floating, this accidental ground will not trip out anything. There may be an RCD on the generator output, but with a single point ground anywhere on the system, it will not trip.

However, the accidental ground on the red phase will result in the neutral being at 240V with respect to ground, and the yellow & blue phases rising to 415V relative to ground.


#138357 - 09/04/03 05:58 PM Re: Generators  
PaulCornwall  Offline
Member
Joined: Jun 2003
Posts: 112
England
Ok lets get this in perspective..

3phase and neutral generator,, RCBO on the supply out.

Earth and Neutral bonded at source..

A fault occurs somewhere on the cables from the generator, as you say, a phase cable trapped in a metal door.

Nothing is going to happen unless the door in bonded to the earth supplied from the generator. I agree..but in therory you wont receive a shock because the generator is not connected to the Mass of earth, even though the door maybe.

So you say take an earth stake from the generator earth and stake it in the ground, in so doing you connect the generator earth to the mass of earth.
Now.. are you saying that this will now, if the door becomes live trip the RCBO??

Well i say it wont,, purely due to the fact that the resistance of the return path to the generator will be to great..

Now i am not the brightest button in the box but i am only trying to get this clear in my head, i still cant see the point..

any one else out there got any ideas on this

Pheww!!


#138358 - 09/05/03 05:51 AM Re: Generators  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
So you say take an earth stake from the generator earth and stake it in the ground, in so doing you connect the generator earth to the mass of earth.
Now.. are you saying that this will now, if the door becomes live trip the RCBO??

Well i say it wont,, purely due to the fact that the resistance of the return path to the generator will be to great..

Taking this situation as you describe it, it will depend entirely upon the overall loop impedance of the circuit, the majority of which will be comprised of the reistance of the ground rod at the generator plus the resistance of the door frame to earth.

If the RCD is a 30mA type, then it will trip if the overall loop is under 8000 ohms.


#138359 - 09/07/03 05:42 AM Re: Generators  
PaulCornwall  Offline
Member
Joined: Jun 2003
Posts: 112
England
Its one of those areas that needs clarifing from the great ones at the IEE.


Cheers anyway



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