There is an upcoming planned power cut, see the attached letter that I got from NPG. It will be for about 28 hours whilst they do some work to our local network and we will be put on a generator.
It says to isolate generation equipment (a heat pump is an electrical generator?!?) from the mains supply, but I'm pretty sure that my inverter will do this anyway when it loses mains power. Plus my PV isolators will not physically close when under load, I can only turn them off at night.
Would turning the trip off in the consumer unit be enough?
Thats how my PV works, it says on the CU to isolate the array in the loft if doing any work on the array, or Id guess doing that at night would be safe 🙂
A great many ground-mounted local substation transformers are equipped with connection points for a generator. I've inserted an arrow in this photo, which is pointing to the connection point for Phase-1 (brown)
To it's left are the connection sockets for Phase-2 (black) and Phase-3 (grey).
Yes, your G98-approved inverter will automatically cease operating once it detects a power-outage. But they're telling you to physically isolate it so that it's not exporting whilst the generator is in operation.
You should have more than one point of isolation. These are usually a rotary switch, and the 32A MCB in your consumer unit.
If your Consumer Unit has an RCBO on the supply to the inverter, then now is a good moment to check if it is bi-directional. An RCBO will trip either because of over-current, or due to "earth leakage". Ie it saves your life if you happen to touch part of the system which has become 'live'.
That's a new regulatory requirement which came into force last year. The usual uni-directional RCBOs have been found to fail when the inverter is exporting back to the grid. The small solonoid coil which causes it to trip can burn-out when left energised.
If you don't know which type of trip you have, then please post a photo here.
This is a safety issue, and it's important to get it right!
Returning to the matter of your planned power-outage, there is likely to be a surge-current, and short-term voltage 'brown-out' when the temporary generator is switched on/off, and again when mains is resupplied from the grid.
Depending on what devices your neighbours have left switched on, the surge might damage electronics. That's why they want everyone to turn OFF high-current appliances with heating coils.
If your landline and broadband are supplied via copper cable, then the surge will also cause fluctuation on the earth wire in your house.
That's bad news for computers and Smart TVs connected directly to your broadband hub. In my experience, the communication interface will manifest an intermittent fault as a result of that earth fluctuation.
Intermittent faults are extremely hard to diagnose!
It's best to remove any copper connection leads from your telephone apparatus and router before NPG start switching over to their generator.
A laptop running from its battery, and connecting via WiFi will be OK. There's no copper cable to be affected by a surge.
This post was modified 2 months ago 3 times by Transparent
That's a new regulatory requirement which came into force last year. The usual uni-directional RCBOs have been found to fail when the inverter is exporting back to the grid. The small solenoid coil which causes it to trip can burn-out when left energised.
Interesting, could this problem also burn an MCB in a setup where an RCD is connected to that MCB to feed the inverter but will also receive power from the inverter back to the board?
We would need to know more about the MCB, RCD and the inverter which is connected, @batpred
In most cases the inverter will be one which shuts itself down in the event of it losing its mains input. Ie it's doing so under the requirements for G98 certification.
Since the inverter isn't now operational, there's no reverse current to damage either the MCB or the RCD.
However, if the inverter is one which connects to a storage battery, and can work in 'off-grid mode' during a power-cut, then there's a risk of the solenoid burning out in the RCD.
Personally I don't like seeing an RCD supplying multiple MCBs in a house which has inverters or EV chargers.
Those devices have a DC-component superimposed onto the AC waveform, and should therefore either have a dedicated RCD with a dedicated MCB, or have an RCBO for each circuit.
That allows you to choose an RCD of Type-A, or preferably Type-B, which have the ability to trip under DC fault conditions.
We would need to know more about the MCB, RCD and the inverter which is connected, @batpred
In most cases the inverter will be one which shuts itself down in the event of it losing its mains input. Ie it's doing so under the requirements for G98 certification.
Since the inverter isn't now operational, there's no reverse current to damage either the MCB or the RCD.
However, if the inverter is one which connects to a storage battery, and can work in 'off-grid mode' during a power-cut, then there's a risk of the solenoid burning out in the RCD.
The inverter is a Solis S6-EH1P8K-L-PLUS, the RCD (100ma)+MCB that I mentioned are dedicated to this grid connection supplying the inverter. All circuits supplying the house loads are under RCBOs.
The EV charger has a dedicated board fed directly from the same "splitter" that also feeds the main board.
Personally I don't like seeing an RCD supplying multiple MCBs in a house which has inverters or EV chargers.
Yup, we got rid of whatever RCDs were there as part of getting a new board that was wired for the inverter and enabling UPS type of functioning. A lot less messy than before and room to grow.
When I said we'd need to know more about the RCD, MCB and inverter, I hadn't expected you to simply throw the model number at me!
However, I have just spent an hour reading the technical specification and installation manual for the Solis Hybrid off-grid models.
Yes, there are circumstances under which a fault could cause the solenoid in the RCD to burn out. You wouldn't know this... except that it would no longer be providing you with protection against death.
... however, since the RCD is rated at 100mA trip-current, you'd be dead before it trips anyway! 🤨
I would strongly recommend that you change the RCD which supplies the Solis inverter to a bi-directional model rated at 30mA trip current.
The terminals should be marked L & N, but not have 'In' or 'Out' labels of course! There should also be a double-arrow logo on the front which remains viewable after the unit has been installed
Proteus were the first UK Company to offer these, and I've discussed the issue with their Business Manager last year. They're reasonably priced, and I see no reason to look any further than this reputable manufacturer.
Please check that your MCB is a double-pole device. If it should trip due to over current, then it should also cut the neutral connection.
If the Neutral were to remain connected to the inverter, then the MCB wouldn't satisfy the Solis definition for this being regarded as a Main Breaker.
Or, of course, you could ditch both the RCD and MCB, and replace them with a Proteus double-pole, bi-directional RCBO!
The Solis installation manual states that the mains input circuit should be protected to 100A.
I'm not entirely happy with that. The fuse rating is dictated by the current capacity of the wire, not the specifications of the appliance it's attached to !
Bi-directional RCBOs are available up to 50A, which is also the maximum current which the Solis 8kW inverter can take.
If your connection wire is sufficiently heavy-duty, then you might prefer to have a 63A Main Breaker. To achieve that, you'd stick with a separate RCD (63A, 30mA trip) and double-pole MCB (63A over-current trip).
As I'm not a qualified electrician I'm not going to provide here the required cable spec for each of those options.
I'd like readers here to employ a properly certified electrician. This is not a DIY matter!
When I said we'd need to know more about the RCD, MCB and inverter, I hadn't expected you to simply throw the model number at me!
However, I have just spent an hour reading the technical specification and installation manual for the Solis Hybrid off-grid models.
I really appreciate you doing this! Solis initially provided a leaflet covering many markets mentioning the RCD sensitivity expected and also how they expected the sizing of the MCBs to be done. To be honest, after my first look and even without knowing what I now know, I thought it was not very clear.
So I had the specs confirmed by Solis and they were made aware this is a UK installation.
Yes, there are circumstances under which a fault could cause the solenoid in the RCD to burn out. You wouldn't know this... except that it would no longer be providing you with protection against death.
... however, since the RCD is rated at 100mA trip-current, you'd be dead before it trips anyway! 🤨
An RCD testing OK and not doing what it is expected to is concerning.
The cables are a short run and indoors.
I really will have this re-checked as the advice Solis gave was also to oversize the cables and MCB, etc ratings by a factor (1.5 I think) and all this was done.
Clearly there are several layers of protection (they advised that the inverter has a built in RCD, which would protect in situation where it is supplying to the consumer unit) but all of these layers need to be in place...
I would strongly recommend that you change the RCD which supplies the Solis inverter to a bi-directional model rated at 30mA trip current.
The terminals should be marked L & N, but not have 'In' or 'Out' labels of course! There should also be a double-arrow logo on the front which remains viewable after the unit has been installed
Proteus were the first UK Company to offer these, and I've discussed the issue with their Business Manager last year. They're reasonably priced, and I see no reason to look any further than this reputable manufacturer.
Please check that your MCB is a double-pole device. If it should trip due to over current, then it should also cut the neutral connection.
If the Neutral were to remain connected to the inverter, then the MCB wouldn't satisfy the Solis definition for this being regarded as a Main Breaker.
Or, of course, you could ditch both the RCD and MCB, and replace them with a Proteus double-pole, bi-directional RCBO!
The Solis installation manual states that the mains input circuit should be protected to 100A.
I'm not entirely happy with that. The fuse rating is dictated by the current capacity of the wire, not the specifications of the appliance it's attached to !
Bi-directional RCBOs are available up to 50A, which is also the maximum current which the Solis 8kW inverter can take.
If your connection wire is sufficiently heavy-duty, then you might prefer to have a 63A Main Breaker. To achieve that, you'd stick with a separate RCD (63A, 30mA trip) and double-pole MCB (63A over-current trip).
As I'm not a qualified electrician I'm not going to provide here the required cable spec for each of those options.
I'd like readers here to employ a properly certified electrician. This is not a DIY matter!
I hope that adequately answers the question!
The Solis Manual discusses cable gauges but it did not have the required precision. I think they caveat the installer needs to spec all this.
The double pole AB0 switch is 100A and part of the connections within the Contactum board, before these MCB/RCD.
Absolutely agree any such circuits should be sized and installed by a certified electrician.
The double pole AB0 switch is 100A and part of the connections within the Contactum board, before these MCB/RCD.
This would mean a lot more to us if we could see photos. 🤨
Every time circuits are altered or added on my set of consumer units I take photos and add lots of pretty coloured labels.
Here's the main set of three consumer units at the end of 2024
And here's a shot of the fourth unit during the installation work. I needed this to show the connection arrangement on some mains filters which are positioned behind MCB's, RCDs and relays.
Please document as much as you can.
It's amazing what you need to refer back to a year later!
None of this is commissioned at the moment. And there were plenty of drawings etc. The goal is for it to be simple for family members that absolutely do not want/need to know to switch it all on, as close to if it was a standard feed from the grid. And hopefully not need to change anything after the power comes back after an outage.
I will post more, but right now we are pressing Solis to review and adjust the inverter settings and also reconfirm their recommendation to not use 30ma RCD. And confirm the "oversizing" as bi-directional RCDs and RCBOs are not available in all sizes.
Like FuseBox has a 50A 30mA RCBO TP+N (switched) 10kA (Type A) B Curve Bi-Directional that their odd 1.5x rule would exclude.
So essentially trying to push them to prioritise the key questions they own and not seek distractions.
we are pressing Solis to [...] reconfirm their recommendation to not use 30ma RCD.
I hadn't realised your reason for a 100mA trip was due to a Solis recommendation.
The 100mA and 300mA trip levels are made available for fire protection. They're not intended to prevent damage to the human body.
It's a tricky issue because even a 30mA trip might not prevent the heart stopping, if, for example, the electrical path taken by the shock-current happened to be straight through the heart.
Designing an inverter so as to avoid all earth-leakage is difficult. Solis might have struggled against a tide of consumer complaints that RCDs were tripping out, and therefore started to recommend that 100mA devices were fitted.
The goal is for it to be simple for family members [to...] not need to change anything after the power comes back after an outage.
That should be readily achievable as far as the inverter is concerned.
However, you might have other appliances in the home that 'trip out' if they're connected to the Solis port which doesn't stay live during an outage. When the grid is re-energised there can be all sorts of spikes and surges which the suppressors in the inverter are unable to quench.
That's not Solis' fault.
And, if you should experience that occurring, then I might be able to help with advice regarding where the Surge Protection Devices (SPDs) are fitted within your consumer unit. The distance between the SPDs and the circuits they protect does matter! So ask here if you want further help with that.
Just wanted to share this link that Solis posted on the sensitivity of the RCDs. To be honest, I will not be looking into it in much detail as just focus on my specific case is hard enough!
But you may have a lot more to say and perhaps influence the powers. A bit more focus from them to advise each market (or perhaps Europe as a whole) may help reduce safety issues.. Or even ideally they will defend the safer standards should be used everywhere..
