Updated 31 January 2023

This blog is from 2014 updated in Feb 2018 and refreshed in July 2022.

A few years ago just before Christmas we had an extended power outage, not only didn’t the telly work :-(,  but all the food in the fridge/freezer was nearly spoiled which would have been a disaster, it was at that point I decided to install an external power inlet point for a petrol generator and some form of switching.

I decided early on that I only needed the essentials to be on the generator backup, this included the heating, lighting, kitchen power circuit and cooking, it was important that I confirmed which circuit breaker control which circuit, this is important as I need to isolate high current consuming circuits so as not to overload the generator when online.

The generator I bought was a Honda 3kVA manual pull start unit off eBay (it later transpired that it wasn’t a genuine Honda, you’ve got to love shysters), which should be plenty big enough, if we need to heat water for hot drinks we’ll use the gas hob kettle rather than the electric one so as not to overload the genny, I can keep an eye on actual consumption due to a digital power monitor which is incorporated into the transfer switch enclosure.

As the generator is a manual pull start their was no point having an automatic power transfer switch, so I built a manual one.

So, outside in the meter cupboard is a 16A switched male socket inlet, this has been modified with a power indicator which illuminates when the generator is running, the generator plugs into this external outlet via small lead, two things to note, first that the petrol generator is outside so that fumes can’t get into the house and secondly that the power lead from the generator to the house uses a female socket to ensure that no exposed pins can be touched with the generator running, removing any shock risk.

From the external socket a 4mm cable feeds into one side of a power transfer switch, this switch has a capacity of 125 amps and is a break before make type, this will ensure that it is not possible to back feed power to the generator from the utility supply during the manual switching operation.

The supply from the utility company also go to this transfer switch, the output of the switch goes to the consumer unit and from here to each of the circuits in the house.

Operation – under normal conditions, the transfer switch is set to Utility supply, this is confirmed by a green 230vAC LED wired directly across the supply via a1A fuse.

On a sustained power outage, the generator is hooked up and started, the switch on the external socket inlet is turned to ON and the Red generator power available LED is lit, (the 230vAC LED is wired directly across the generator supply at the switch via 1A fuse).

Non essential circuit breakers in the consumer unit are turned to the OFF position, once done, the transfer switch generator power available LED is checked, and if still available, the transfer switch is operated to import power to the consumer unit from the generator.

As the transfer switch is in the garage, I would not know if the Utility power had been restored, I therefore fitted a 230vAC buzzer across the Utility supply via a switch, restoration of utility supply is indicated by an audible tone which obviously is switched OFF in normal Utility power operation.

To check that the generator is not being electrically overloaded, an ACM20 panel meter was fitted, this displays, amongst other things is power (Watts) being drawn, which is very useful for this monitoring function.

System on test, instruction by the consumer unit, give the start-up and shutdown process including which breakers to turn off before changing over, the transfer switch panel meter is used for monitoring load to make sure the generator is not overloaded.

Update

17th December 2022 – I’ve noticed over a few months that the ACM20 display has been acting weird, and today it finally gave up giving me believable readings, so a new one is on order from Mouser.co.uk for the sum of £80.47.

31 January 2023 – Taken a while but finally got round to replacing the original meter, the details of the new one are:

• www.murata-ps.com
• ACM20-4-AC1-R-F-C
• Part Number – 4900204

The only issue I had was getting the Current Transformer wires the right way round as I had to extend them, you know if its wrong when the Wattage stays at zero!