Generator setup and earthing

[Sandman34]

Hi all new to the forum and electrican. Working on a plant at the moment where the client is wanting to have two 480volt 60hz three phases motors installed inside there workshop. These motors are going to be fed from a generator as they do not have enough power to feed them off the workshop supply. So generator provided and a distribution board installed inside the workshop for isolation and protective devices for the two motors. The supply at the workshop is 415volt 50hz three phase and neutral. Now just have a few questions before I start.
1. I would like to run the motors at there nameplate rating so would set the generator up to supply 480volts 60hz. Will this be ok considering that the workshop supply is 415volts 50hz?
2. The generator will be sited outside the workshop and for earthing I am looking to do as follows. On the generator main switch generator side neutral connection has an Earth cable from there to the frame of the generator. I was planning on taking a Earth from this point half the size or more of the phase conductor to the distribution board sited inside the workshop then from the distribution continue the earth to the M.E.T of the workshop this will get the neutral grounded and also having a Earth at my distribution board. Then there are Earth bosses either side of the generator in the lifting frame which the lifting frame is bolted to the the generator frame. So I am going to take half the size or more of the phase conductor from one of the earth bosses to the M.E.T inside the workshop. Will this set up be satisfactory?
3. Setting of the MCCB of the generator. Generator has a 630amp MCCB. I am only needing to run 185mm cables from the generator. So settings of MCCB is as follows
- Io= In * 0.5 = 315amps rating is the breaker now
Ir= Io * 0.8 = 252amps trip point
Isd= Ir * 8 = 2016amps for starting inrush current
Motors are both 55kw. Is this set up correctly.
4. Generator has a RCD inside the generator. I am not taking a neutral out of the generator as motors do not require it my RCD at 30ma will still work as have link between Earth and neutral that is inside the generator connection from neutral gen side the the frame of the generator.
5. As limited Earth connections for on the distribution board that I have supplied with the cable I am installing planning on having the Earth cable from generator onto the earth boss inside the dist board. Then linking 0.5 or more of the phase conductor from that Earth boss to another Earth boss on the distribution board which is an Earth bolt then from there taking 0.5 or more of the phase conductor from outside the enclosure onto this same bolt to the M.E.T of the workshop. Is this satisfactory?

Is this set up all satisfactory? Sorry for the long message be much appreciated if can answer my questions.

Kind regard

 

[Darkwood]

Welcome to the forum

 

[Sandman34]

hi dark wood thanks but do you know would this be suitable.

regards

 

[davesparks]

What you are going to be using as the actual connection to the general mass of earth for this generator? It's looks to me like you are planning to use a DNO earth connection for this, which is far from acceptable.

Why are you sizing the earthing conductors based on 0.5x the phase conductor, why not actually calculate the size required, especially considering that the fault current available from a generator is likely to be nowhere near that available from an equivalent DNO.

Is 30mA RCD protection really suitable? I would have thought a somewhat less sensitive earth fault protection would be more appropriate.


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[Sandman34]

Hi Davesparks why is it unacceptable to use the DNO Earth for connecting to the generator. Around where the generator is getting sited there is no where to get a rod installed or earth boss? I am just sizing earthing to 0.5 as that's the way I have been taught I could calculate it but just stick to 0.5 of phase. Yeah 30ma is adjustable so can put it to anything from 30 to 300ma if I want but not tripping on 30ma so just keeping as that.

Kind regard

 

[LeeH]

What happens If the supply cable is broken for maintenance by the DNO????

 

[Brightspark2]

Hi Davesparks why is it unacceptable to use the DNO Earth for connecting to the generator. Around where the generator is getting sited there is no where to get a rod installed or earth boss? I am just sizing earthing to 0.5 as that's the way I have been taught I could calculate it but just stick to 0.5 of phase. Yeah 30ma is adjustable so can put it to anything from 30 to 300ma if I want but not tripping on 30ma so just keeping as that.

Kind regard

It's in the British standard, the generator must have its own connection to earth and not the DNO.

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[Sandman34]

I wouldn't have they would be running motors inside the plant in the dark. As there will be no supply to workshop so no power or lighting. Risk assessment and all working in the dark?

 

[davesparks]

Hi Davesparks why is it unacceptable to use the DNO Earth for connecting to the generator. Around where the generator is getting sited there is no where to get a rod installed or earth boss? I am just sizing earthing to 0.5 as that's the way I have been taught I could calculate it but just stick to 0.5 of phase. Yeah 30ma is adjustable so can put it to anything from 30 to 300ma if I want but not tripping on 30ma so just keeping as that.

Kind regard

It is unacceptable because it could result in a DNO worker receiving a fatal electric shock if they are working on their system with no knowledge of this connection. At which point you will be guilty of either murder or manslaughter.
Do the job properly and don't place people's lives at risk.

How do you know that sticking with 0.5 of the phase conductor is correct for this installation, generators are rarely capable of delivering high fault currents due to the shear mechanical energy required to create the fault current. I would calculate it properly every time with a generator supply.

When you say it's not tripping are we to assume that you have already commissioned this installation and it is in service?


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[Sandman34]

Hi Davesparks ok then generator is not commissioned yet. So now then I can't get a rod installed around the other side of the workshop fair but away though. But will do this by rodding at this point run Earth cable from this rod to my newly installed distribution board inside the workshop. Then from this distribution board run my supply cable from my generator 185mm 4core (three phase and Earth) with the earth of the 4 core connected to the earth boss connected to the frame of the generator and also linked to neutral of the generator. Then take a bonding conductor from the distribution board or Earth rod to the lifting frame of the generator. Is this acceptable? Thanks for getting back

kind regards

 

[davesparks]

A rod? are you sure a single rod will be enough? Have you done any resistivity tests to predict what earthing will be required or established what value of resistance to ground you will need to achieve for this earth connection?

Why can't you install electrodes local to the generator? Is it indoors or something?

You will almost certainly still need to link the earthing of this generator to the earthing of the normal electrical installation, but that will have to be designed to comply for the most onerous fault condition possible for either of the supplies.


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[Sandman34]

Hi Davesparks its on concrete around the other side is soil. I can install my rods there. When I work out how many I will need. Then run Earth cable from these rods to the dist board for motors in workshop. Then from this dist board run my cable to the generator. Linking generator to the workshop supply can do this from dist board to where I need to go. How would that work?

kind regard

 

[davesparks]

I would be concerned about installing the rods remotely from the generator, but I'd need to sit down with the reference books for a while and have a lot more information to know whether my concerns are a real problem or me going over the top.
This might be a daft question but why can't you drill through the concrete to install the rods? You might even find the rebar to connect to and give a good earth if the concrete is a big enough lump


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[brenfox]

I would be concerned about installing the rods remotely from the generator, but I'd need to sit down with the reference books for a while and have a lot more information to know whether my concerns are a real problem or me going over the top.
This might be a daft question but why can't you drill through the concrete to install the rods? You might even find the rebar to connect to and give a good earth if the concrete is a big enough lump


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Interesting subject. (mainly because I do not know much about installing generators!). I know the regs state that the generator requires a separate supply but can you explain why using the DNO earthing supply could be potentially lethal? cheers.

 

[davesparks]

Interesting subject. (mainly because I do not know much about installing generators!). I know the regs state that the generator requires a separate supply but can you explain why using the DNO earthing supply could be potentially lethal? cheers.

I don't think the regs make any such statement, but then I've never actually looked for it.
I think it's more of a case of it is lethal rather than being potentially lethal as I believe there have been incidents of DNO workers being electrocuted as a result of this.

The DNO earth is only reliably connected to earth when the network is fully connected and operational. The common use for generators in an installation is as a backup in the event of a powercut, so it's fair to say the generator will be in operation when the DNO earth connection may very well be disconnected at some point in the network, and so the generator star point will be left floating, and the entire installation at risk.
Also as a result of this the unlucky DNO worker may find himself caught between the two ends of the disconnected earth, one of which is at somewhat higher potential than expected thanks to the generator. This could result in an unexpected arcing when they make a connection, or an electric shock.

Obviously in this particular thread the generator is forming an additional supply rather than a backup supply, but a situation could still occur where the generator is running during a network fault.


Ultimately it is not a very good idea to rely on an earthing connection which is not under your control for your own means of generation like this.




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[LeeH]

Interesting subject. (mainly because I do not know much about installing generators!). I know the regs state that the generator requires a separate supply but can you explain why using the DNO earthing supply could be potentially lethal? cheers.

For the reason I said, what happens if the DNO disconnects it.