Standby generator questions

[JohnW2]

You would do better to have a parallel independent system in your home for use on the rare occasions when there is a power cut. People with a standby generator often have a red socket next to the boiler so their heating system can be plugged in.

That's precisely the arrangement I have in my house - a totally different wiring system feeding a number of (mainly red) sockets all over the house (including by the CH control system, which is fed via a 13A plug) and also a series of 'permanently on'(when genny is on) lights at strategic points all over the house. In the event of need, all of that, but nothing else, is fed by the genny.

However, in relation to erics comments and my responses, there is no practical way in which I could avoid the genny's 'earth' being connected to the DNO's one (if I had TN,, which I don't).

Kind Regards, John

 

[JohnW2]

Some vital services surely have a requirement that power cannot be turned off even for a second. Hospitals I believe have emergency generators, and some means of switching instantaneously to an emergency supply? Otherwise what happens to patients on life support when the power suddenly fails ?

Crucial equipment in hospitals such as you mention are usually supplied via dedicated circuits (often using 'non-standard' plugs/sockets).

Since a large genny cannot be up-and-running in "less than a second", such circuits will usually have some sort of battery-powered 'UPS' to cover the changeover period.

Kind Regards, John

 

[JohnW2]

I will admit this also seems odd to me, as clearly I can fit an EV charge point as TT, or the garden shed. However a valid method to find the loop impedance is inquiry, the DNO with a TN supply are required to tell you the ELI, and also what earthing system you have. ...

If they know - and it seems that they often don't (without inspecting your property).

I have seen it where the DNO has allowed an odd property to use TT, an example was the radio ham who's earth wire to shack melted when the PEN was lost, they did not like it, but had to under the circumstances give permission for house to be TT.

I don't think nit works like that. A DNO can 'offer' a TN earth (as they have to me),but one is free to decline that offer and have one's own TT earth if one wants. That's certainly what I've done, and the DNO guys I've discussed that with (when 'declining' their offer of TN-C-S) don't seem toi have had a problem with that.

With the gas terminal I worked on, earthing was very important, and there was a resistor the size of a small car between DNO earth and site earth to limit current flow.

Are there not safety implications in deliberately having resistors in an earth path?

Kind Regards, John

 

[ericmark]

Are there not safety implications in deliberately having resistors in an earth path?

It was a very low value, likely less than a ohm, and simply limits current to what cables can handle. But a resistor must be safer than a diode and we put diodes in the earth path for galvanic protection.

 

[Jackrae]

With the gas terminal I worked on, earthing was very important, and there was a resistor the size of a small car between DNO earth and site earth to limit current flow.

That may have been the site generator(s) star point current limiting resistor, chosen to limit the fault current in the event of a phase-to-ground fault. The generator(s) for such a resistor was probably rated in the Mw range.

On such a terminal you would probably have had both 'power' earths and 'IS' (intrinsically safe) earths, which added a degree of complexity as to what "earth" you chose when connect devices.

 

[flameport]

've come across those blue caravan hook up type connectors and I understand they are splash proof so probably not good enough for an exposed outside wall

They are suitable for use outside - that's why they are used for caravans, which generally spend all of heir time outside in all weathers.

Unless you want to spend significant sums to have a generator connection, don't bother.

Power cuts are rare, and the suitcase type of generators are only intended to supply one item of equipment.
Connecting a generator to a fixed installation properly isn't trivial, and won't be cheap.
There is also the consideration of the fuel for this generator - petrol will be useless after a month or two in storage, and if left in the generator will quickly gum up the whole thing making it useless.

If you just want something to run the CH boiler controls, internet and a few other bits, then something like this: https://www.amazon.co.uk/dp/B08RNT5BDT/ will be a far better option, as no fuel required, it can be used inside, and won't need any maintenance.
It's also cheaper and safer than connecting a generator to the fixed wiring.

 

[Nozzle]

Huh? Feed-in power is synchronised to the grid frequency - as it clearly has to be.

Embeded generation isn't 'dispatchable' by National Grid, it just appears as a reduction in demand. As cloud cast shadows over the domestic panels and get moved around countrywide, the "apparent demand" therefore goes up and down even if actual demand is steady.

That's how embeded generation effects grid frequency - (in both directions)

Nozzle

 

[JohnW2]

It was a very low value, likely less than a ohm, and simply limits current to what cables can handle.....

Yes, I realised that, but isn't it rather back-to-front in terms of normal design principles? Should one not install cable capable of handling the PFC, rather than trying to limit the fault current through an 'undersized' cable (and thereby also increase the PD between the installation's 'earth' and true earth in the event of a fault)?

Kind Regards, John

 

[YorkshireMidge]

They are suitable for use outside - that's why they are used for caravans, which generally spend all of heir time outside in all weathers.

Unless you want to spend significant sums to have a generator connection, don't bother.

Power cuts are rare, and the suitcase type of generators are only intended to supply one item of equipment.
Connecting a generator to a fixed installation properly isn't trivial, and won't be cheap.
There is also the consideration of the fuel for this generator - petrol will be useless after a month or two in storage, and if left in the generator will quickly gum up the whole thing making it useless.

If you just want something to run the CH boiler controls, internet and a few other bits, then something like this: https://www.amazon.co.uk/dp/B08RNT5BDT/ will be a far better option, as no fuel required, it can be used inside, and won't need any maintenance.
It's also cheaper and safer than connecting a generator to the fixed wiring.

With caravans and RVs (I'm not a user of either) it just seemed to me that they tended to be recessed to protect them which would also keep the worst of the weather off too, so that was all I was querying.

The power unit you linked to is an interesting bit of kit - but with having the gen and a UPS already, appeal is a bit limited as is the run time. Fuel, I'm less worried about as our mower is petrol too, so it does tend to get refreshed and I don't let petrol sit in the gen either (it's easy to drain the carb on the gen, as I do for the mower carb over winter).

The reality is if its a short duration outage, I wouldn't even bother dragging the gen out anyway. What got me thinking this time, was the bang from out in the street was so loud, it woke my dad up who lives on another estate about half a mile away, and it was followed shortly afterwards by a Police van and fire engine. We thought something had happened to the sub station and we might be facing a lengthy outage - so it was quite a relief when they got it back on as fast as they did esp. given recent storms, staffing challenges through covid etc.

Agree, outages ought to be rare, but as I mentioned, this is the second several hour outage where the road has had to be dug up inside 12 months, and we've had short periods of outage over a few years prior.

Anyway, there seems to be a consensus from everyone that this is a bit more difficult than it at first appeared, so I think I'll keep things basic and revisit if anything else happens in the next few months.

Thanks all.

 

[BS3036]

what makes it more dangerous than solar power?

The problem to the user these days tends to be low volts/wobbling frequency due the the amount of feed in power without a frequency reference.

As cloud cast shadows over the domestic panels and get moved around countrywide, the "apparent demand" therefore goes up and down even if actual demand is steady.

I find it hard to see how a very tiny variation in net demand, which averaged over the whole country it will be, (except during a solar eclipse) could be described as 'dangerous' or resulting in 'low volts/wobbling frequency due the the amount of feed in power without a frequency reference'.

 

[Harry Bloomfield]

Some vital services surely have a requirement that power cannot be turned off even for a second. Hospitals I believe have emergency generators, and some means of switching instantaneously to an emergency supply? Otherwise what happens to patients on life support when the power suddenly fails ?

Those systems have a UPS to keep them going for a while.

 

[flameport]

We thought something had happened to the sub station and we might be facing a lengthy outage

In those circumstances the network operator will usually install their own temporary generator, which is typically a large truck parked in the street outside.
Exceptions being for extremely remote areas where access isn't possible due to snow or similar.

 

[Nozzle]

I find it hard to see how a very tiny variation in net demand, which averaged over the whole country it will be, (except during a solar eclipse) could be described as 'dangerous' or resulting in 'low volts/wobbling frequency due the the amount of feed in power without a frequency reference'.

You're right, not dangerous.

 

[AdrianUK]

Yes, I realised that, but isn't it rather back-to-front in terms of normal design principles? Should one not install cable capable of handling the PFC, rather than trying to limit the fault current through an 'undersized' cable (and thereby also increase the PD between the installation's 'earth' and true earth in the event of a fault)?

Kind Regards, John

I think the problem here is that 'impedance earthing' is being taken out of context.

Its normally done, in controlled conditions, to limit the fault current & hence the damage to a piece of equipment, usually a generator or a transformer.

A typical example would be a large (several MW) genset. The output of which would be connected to a delta/star setup transformer. The generator star point being connected to earth via a resistor. The value of this resistor would be chosen to limit the magnitude of an earth fault in the generator stator to such a value that it doesn't cause (more) damage but can still be detected by the protection equipment. This might, typically, be 1/10th of the full load current. This will minimise the damage at the point of the fault and can make the difference between the equipment being destroyed or repairable.

This is all DNO/Distributor/Generator land stuff - 'IT' systems are not allowed on the public network.

Adrian

 

[JohnW2]

Those systems have a UPS to keep them going for a while.

Indeed......

Crucial equipment in hospitals such as you mention are usually supplied via dedicated circuits (often using 'non-standard' plugs/sockets). .... Since a large genny cannot be up-and-running in "less than a second", such circuits will usually have some sort of battery-powered 'UPS' to cover the changeover period.

Kind Regards, John