Generators

[ElectricDunc]

Got a few questions regarding generators. I installed one a good few years ago but it was nowhere near the size of this one. The proposal on this job is to install a large 165kVA generator for day use to run heavy loads in a packing plant as well as living accommodation. There will then be a switchover at night on to a smaller 40kVA generator which will only power the living accommodation once the packing plant has shut down. Reason being the big generator isn't happy running at lower loads. There is no mains supply to this premises btw.

I know the generators have to be earth staked but i'm unsure of what size cable to use between the generator and the stake. Also with regards to the changeover switch what size will it have to be. If i've worked it out right it should need to handle somewhere in the region of 270A. Are there changover switches out there that will handle being operated under load. Also how will the motors in the packing plant take the on-off-on and slight phase shift experienced on switchover.

Just looking for some advice as this is out of the normal scope of work i undertake. I have worked with loads as high as this before so understand the importance of safety and proper connection of terminals however i am not au fait with everything relating to this subject. Just one final note, the generators will not be running in tandem so i don't need to worry about getting them running perfectly in phase with each other though i will obviously confirm phase rotation is correct before bringing everything online.

Not a DIY subject i know but there are electricians on here who work in the industrial sector who may be able to help.

 

[ericmark]

Got a few questions regarding generators. I installed one a good few years ago but it was nowhere near the size of this one. The proposal on this job is to install a large 165kVA generator for day use to run heavy loads in a packing plant as well as living accommodation. There will then be a switchover at night on to a smaller 40kVA generator which will only power the living accommodation once the packing plant has shut down. Reason being the big generator isn't happy running at lower loads. There is no mains supply to this premises btw.

I know the generators have to be earth staked but i'm unsure of what size cable to use between the generator and the stake.

I is normally around the 70mm mark although in real terms it would be hard to get an earth good enough to take that current so one is normally looking at RCD protection.

Also with regards to the changeover switch what size will it have to be. If i've worked it out right it should need to handle somewhere in the region of 270A. Are there changover switches out there that will handle being operated under load.

There are change over switches that will take the load and I have used them abroad but never needed anything that big in UK

Also how will the motors in the packing plant take the on-off-on and slight phase shift experienced on switchover.

It is normal if not synchronising to give 5 minutes between switching of one generator and switching on another mainly for fridge/freezer considerations they do not like starting under load so one allows time for pressure to drop before re-energising

Just looking for some advice as this is out of the normal scope of work i undertake. I have worked with loads as high as this before so understand the importance of safety and proper connection of terminals however i am not au fait with everything relating to this subject. Just one final note, the generators will not be running in tandem so i don't need to worry about getting them running perfectly in phase with each other though i will obviously confirm phase rotation is correct before bringing everything online.

Not a DIY subject i know but there are electricians on here who work in the industrial sector who may be able to help.

Where I had a similar change over on supply to cranes where when working they were powered from a 250KVA CAT generator and at night the air craft lights were powered from a 63KVA we used a simple plug and socket and only moved the lights across and left crane connected. Since you would not want the factory load going to small generator you may wish to doing something similar and only switch domestic and lighting and leave the main bulk permanently connected to main unit. This would also reduce switch size.
Often the switches are just lockable switches with a single like key so only one can be switched on at a time. The idea of a single switch is great until you realise what can happen if power is returned too quick so often some means is devised so the operator has to take some time between switch off and switch on. One job the switches were on opposite sides of the shed to delay time between switch over. If you don't want a delay then you must synchronise either you can't have simple system and no delay.

 

[ban-all-sheds]

2 generators anyway.

Why not one just for the plant, and one just for the accommodation?

 

[ElectricDunc]

Good question, i assume it's only been designed this way to save on fuel.

 

[ban-all-sheds]

When you're into tens and hundreds of kVA, won't most of the fuel consumption be determined by the electrical loading?

 

[ricicle]

Good question, i assume it's only been designed this way to save on fuel.

If this install has been 'designed' (which I would hope so by a suitably qualified Electrical Engineer) then surely the cable sizings would have been worked out

 

[Spark123]

Got a few questions regarding generators. I installed one a good few years ago but it was nowhere near the size of this one. The proposal on this job is to install a large 165kVA generator for day use to run heavy loads in a packing plant as well as living accommodation. There will then be a switchover at night on to a smaller 40kVA generator which will only power the living accommodation once the packing plant has shut down. Reason being the big generator isn't happy running at lower loads. There is no mains supply to this premises btw.

I know the generators have to be earth staked but i'm unsure of what size cable to use between the generator and the stake.

I is normally around the 70mm mark although in real terms it would be hard to get an earth good enough to take that current so one is normally looking at RCD protection.

As the earth spike will be the "Means of Earthing" then RCD protection isn't necessary for this.
iirc this means of earthing needs to have a resistance of less than 21 ohms to earth using method one in GN3 (3 or 4 wire test).