Power cuts and loss of earth

[Tim+]

My solar/battery system has connections for an emergency power supply system. We didn’t have it connected up as power cuts are very rare in our neighbourhood, for now at least.

Anyhow, to wire it up requires an independent earth system to be installed. I understand the requirement *but* if one is supplied by an underground cable and the local substation is literally across the road what are the chances of losing our earth connection without a JCB digging up the road between our house and the substation?

If I were to connect it up it would only be to supply our fridge freezer and a few lights, not feeding into the house wiring at all.

Yes, illegal I’m sure but how unsafe would it be to use our existing earth given our circumstances?

Tim

[Smolley]

I thought your battery would be the emergency power system.

[Owain Lastname]

Are you 100% sure your supply comes directly from that substation ?

I don't know how earths are terminated in substations but there has to be the possibility that it might get disconnected if cables are moved or equipment is replaced and your cable is left poking out of the ground until a new transformer is dropped on site.

Owain

[charles]

The company that installed my Solar Panels & battery has provided a changeover switch to allow me use my panel& battery to power the house if needed. Nothing has been done to the earthing arrangements in the house - and why should it have been?

[Fredxx]

That was my thought too. In fact having different appliances on different earths in proximity to each other sounds a recipe for disaster.

[Tim+]

It would be, but according to regulations it needs its own earth system (I.e. earthing rods).

Tim

[Tim+]

Because an independent earthing arrangement is a requirement in the regulations.

Tim

[charles]

Do tell me which regulation. I've BS7671:2018 in front of me.

[Tim+]

It’s in the instructions for my inverter. I assumed it was the same for all. The reasoning as I understand it is that one needs an independent earth system in the event of power loss as one might have lost the power company’s earth (however unlikely that might be).

Tim

[John Rumm]

Because the supplier does not guarantee that their earth will continue working during a power cut. Obviously in the vast majority of cases it will still work, be but you can imagine cases where it may fail. i.e. TN-S earth disconnected at the substation, or TN-C-S earth failing because of a severed PEN conductor on the supply[1]

[1] PME normally adds additional earth connections along the route to mitigate that possibility since the dangers of losing Earth and Neutral at the same time at the property are more serious.

[John Rumm]

They would not be on different earths as such since you would add your TT earth to the supply TN-C-S one. In normal operation your earth would just become one of the "Multiple" of PME. If you were to lose the supplier PEN conductor, then you still have your local earth.

This has functional as well as safety implications. For example you may find that a gas boiler will not light when powered from a local supply unless there is a neutral earth bond present. (many use flame rectification to sense when the boiler has successfully lit)

[Theo]

I guess it's something like this. Suppose your supply comes via an overhead cable (which might not be your immediate house feed, but somewhere upstream). You are on TN-S or TN-C-S so your earth comes in via this cable. Hurricane Wotsit comes through and blows down this cable.

Now you have no earth. Suppose there is then a fault that causes the chassis of some appliance to go live. The chassis is connected to earth, but the earth isn't connected to anything, so the chassis can go live without blowing any circuit protection devices.

The inverter ties neutral to earth when in island mode so the protection devices still work (current path live->chassis->Earth->neutral) but I imagine it still needs an earth rod in case some part of that path is through earthed metalwork (eg a faulty greenhouse heater: heater live->heater casing->metal greenhouse->soil->earth rod->Earth->neutral) otherwise such a fault would cause the greenhouse to be live without tripping the protection device.

Theo

[Theo]

Obligatory John Ward video, see 20:07 for discussing the situation of adding a TT electrode to an existing TN-C-S system:

[John Rumm]

551.4.3.2.1

[Tim+]

Can you explain this bit John?

“In systems where a low voltage supply is provided to the installation, the neutral of the supply is earthed at the distributor’s transformer.

Accordingly, in systems operating in island mode, the distributor’s neutral-earth link cannot and must not be relied upon, as this is switched out when the live conductors are disconnected.”

Does this mean that the earth connection in the substation is disconnected during a power cut?

Tim

[John Rumm]

I think some of this working comes from "IET Code of Practice for Electrical Energy Storage Systems" - a doc I don't have to hand, so I am not suite sure exactly what they are describing.

I think the design aim it to make the grid appear as a TN-S earthed system to remove any complications of the locally bonded (to neural) earth of a TN-C-S system looking like a parallel neutral path when operating from local power. There hence tripping RCDs when the incoming supply is switched out.

I think this comes from the slightly more complicated switching employed with "island mode" installations.

Have a look at:

You have to allow for the possibility that it might be - but it does not mean that it normally will.

[Fredxx]

Does that really say "Live". Neutral is considered a "Line" conductor.

No, it shouldn't. But I would agree if the substation was having work carried out then I suppose it is possible for a PEN to float.

[John Rumm]

Neutral should be treated as live conductor, but not a line (phase) conductor,

See

[Fredxx]

Under fault conditions that I have experienced, the TN-C-S earth is a very low resistance that would overcome any purpose of a local TT earth.

So very little point, unless your C-S became O/C?

Won't that be using case as the earth and a local potential WRT case? Do any boilers rely on direct Line to Earth current for flame detection?

>

[Fredxx]

Thanks for the info and link. Both conductors are live, but only a phase conductor is a Line conductor. My memory has failed me where I thought it was the other way round!