UPS via generator. Advice needed

[Roberts7286]

Hi All

Had an enquiry about installing a generator fed UPS to a house. I have a minimal amount of experience with UPS but am certainly no expert, the ones I've been involved with were manual changeover and didn't have anything attached (generator was to be hired in the event of a failure).

There's a very good chance I won't be getting involved in this job as it seems a bit like a can of worms but thought I would see if I could increase my knowledge in this area.

The customer is purchasing a Bohmer-AG WX-7000K-e 7.5Kw generator and intends to use it to run lights and a small number of appliances (fridge/freezer/kettle) in the event of a power failure. They would like the changeover switch (manual) to be internal at the property and a commando socket external to plug the generator into. They would also like to be able to switch the generator on via a key fob or similar so they don't have to go outside to turn it on. I recommended an automatic changeover but they don't want to leave the generator connected permanently.

My questions are:

Can this generator be switched on remotely?

Am I right in thinking that this generator will only give 16A at each of the socket outlets? So the customer won't get the full 7.5Kw if connecting to a single commando socket.

Would the generator start up automatically if connected to an automatic changeover UPS?

Would this generator require a TT earth rod?

Thanks in advance.

Tom

​

 

[Rockingit]

Hi All

Had an enquiry about installing a generator fed UPS to a house. I have a minimal amount of experience with UPS but am certainly no expert, the ones I've been involved with were manual changeover and didn't have anything attached (generator was to be hired in the event of a failure).

There's a very good chance I won't be getting involved in this job as it seems a bit like a can of worms but thought I would see if I could increase my knowledge in this area.

The customer is purchasing a Bohmer-AG WX-7000K-e 7.5Kw generator and intends to use it to run lights and a small number of appliances (fridge/freezer/kettle) in the event of a power failure. They would like the changeover switch (manual) to be internal at the property and a commando socket external to plug the generator into. They would also like to be able to switch the generator on via a key fob or similar so they don't have to go outside to turn it on. I recommended an automatic changeover but they don't want to leave the generator connected permanently.

My questions are:

Can this generator be switched on remotely?

Am I right in thinking that this generator will only give 16A at each of the socket outlets? So the customer won't get the full 7.5Kw if connecting to a single commando socket.

Would the generator start up automatically if connected to an automatic changeover UPS?

Would this generator require a TT earth rod?

Thanks in advance.

Tom

​

Lots to unpack here....

Can this generator be switched on remotely?
The vast majority of smaller gensets can all be remote started either electronically or via RF (haven't looked this one up). However - remember that they all need to be periodically run-up to maintain life and battery levels.

Am I right in thinking that this generator will only give 16A at each of the socket outlets? So the customer won't get the full 7.5Kw if connecting to a single commando socket.
Correct. 16A is basically 4kW. You could always split the circuits but this gets messy to do safely.

Would the generator start up automatically if connected to an automatic changeover UPS?
Only if the UPS in question has a trigger function to do that.

Would this generator require a TT earth rod?
Can of worms, here. Small generators tend not to have solid N based earthing and you'll find many threads on here about this.

Conventional wisdom (BS7430) says that you tie all the earthing together at the MET, however, this doesn't really work in these lash-up systems as there's no real stability anywhere and also the UPS effectively acts as an isolating transformer as well (type depending). So subject to not knowing even 10% of the likely design on this I suspect that what would need to happen would be a common earthing system Gen > UPS > MET all tied to a TT reference point, with the normal incoming distributors Earth (TN-x) being disconnected as part of the switchover for safety.

 

[pc1966]

Typically a proper Automatic Transfer Switch will also provide a 12V charger supply to keep the generator battery ready for starting, but that becomes expensive and not commonly used with small stuff.

You mention a UPS and a generator, how are they planning on using this? Will the UPS be fed normally from the mains supply from the CU as for any common item, or are they planning to have:

Mains --------->|
---------------------|-> Transfer switch -> CU
Gen -> UPS ->|

As @Rockingit says many UPS run as isolated supplies if the incoming power fails, though normally if on-supply there is N continuity. You would have to check that aspect carefully, same as some smaller generators have a floating output though usually larger ones have a N-E link internally so they act as a typical TN supply.

In any case you should have an earth rod as if there is a loss of supply it could well be a total cable cut and so your supply means of earthing is gone as well. Connecting a rod to the existing supply earth at the MET is not a problem, no different electrically from a metal service pipe.

What is tricky is how to deal with the N-E link when you change over to the backup supply. If the generator is isolated then you could link N-E at the UPS output (if it is only fed from the generator, as my rubbish ASCII art above attempted to show) and then you get TN supplies in both transfer positions.

 

[Lucien Nunes]

Would this generator require a TT earth rod?

To clarify this point, connecting an earth rod to a generator does not make it TT. DNO's mains supplies all have an earth electrode at the source (e.g. substation or pole-mounted transformer) to create an earth reference for the distribution system and provide a return path from the mass of earth to the neutral point. The generator's rod serves the same function.

A consumer's TT earth rod is different; it picks up that earth reference and uses it to earth an installation. Only the mass of earth connects the consumer's rod back to the electrode at the substation.

With an onsite generator, the installation MET will invariably be earthed to the generator via the supply cable, making it a regular TN supply, rather than by a second independent rod.