Grid switches on 32A ring

[WiringPeter]

I've been trying to work out whether grid switches (ie. a 20A switch) are acceptable on a normal 32A ring. I've read many threads online. Very few people seem sure whether it's acceptable but many sparks seem to do it in kitchens etc.

The argument seems to be that the switch is rated at 20A but there could be 32A flowing through the ring. However, how much of that 32A would actually go through the terminal? The majority of the current would go straight from one wire and into the other as they will be touching. I can't see it being an issue but the real question is whether it meets regs. Does anybody have the official answer to this?

 

[Andy78]

Have a read up on the regs surrounding ring final circuits with the start off point being that 2.5mm cable clipped direct is only rated for 27A.
Also think about how the 20A DP switch is to be connected into the circuit and the load that would be drawn through it.

 

[Toneyz]

If a bank of grid switches supplying a few high current used appliances this should be located as close as practically possible to the midpoint of the kitchen ring final circuit.

 

[Taylortwocities]

Your grid switch will be on a spur (LOAD) output from a ring

So that will not be 32A, will it? The only place where it might be anything near 32A is at the MCB in the CU.

 

[ChrisElectrical88]

The load across the switch will be 13A maximum as you would imagine that they feed a single socket or spur each.

However i am not a fan of them and have been called out to a few crackling due to poor termination. Takes a lot of care and a 47mm backbox to get them perfect, something not many people do.

 

[WiringPeter]

Have a read up on the regs surrounding ring final circuits with the start off point being that 2.5mm cable clipped direct is only rated for 27A.
Also think about how the 20A DP switch is to be connected into the circuit and the load that would be drawn through it.

That all makes sense now. This is what I had originally though but my mind had been led astray by some convincing replies I had read.

 

[telectrix]

grid switches...spawn of the devil. rated at 20A yet burn out on 13A max. .... need to knock a brick out to get all the cables in. never can get the faceplate on right.... and the 1 marked fridge controls the dishwasher.

 

[WiringPeter]

However i am not a fan of them and have been called out to a few crackling due to poor termination. Takes a lot of care and a 47mm backbox to get them perfect, something not many people do.

This is my biggest fear. However, they look great when they are done well and will take up a lot less space in my small kitchen.

 

[Dustydazzler]

Me personally not a big fan of grid logic switches

But if done correct do serve a purpose

but as telexctric mentions the amount of wonky badly installed ones is frightening , and never assume they are wired correct

turn off the one marked fan and you probably just isolated the washing machine

 

[ackbarthestar]

Nothing wrong with grid switches as long as an electrician is left to make the connections and the accessories are not from poundland.

 

[Taylortwocities]

I really do not like a bank of grid switches fed from a ring final. The whole point of a ring final is to distribute the loads round the ring. With a grid bank you’ve got all of the high power items coming from a single point on the ring.

If you MUST have a mission control panel like this, then it is best fed from a 32A 4mm radial. That finishes the sermon on the mount for today.

 

[APE37]

I am not a fan of this method. The last few firms that I have worked for do this method, even having a fuse holder in the grid switch along with the socket by the appliance, so having 2 fuses in series and making it more fiddly, along with them mounted in a cupboard. Also as has been said before they are fiddly and prone to issues. Especially when under pressure to get jobs done as quickly as possible.

 

[Midwest]

I think the only reason these are used in new builds, is to comply with Part M, with all the sockets for appliances in cupboards or behind appliances.

 

[Deleted member 112675]

Your grid switch will be on a spur (LOAD) output from a ring

So that will not be 32A, will it? The only place where it might be anything near 32A is at the MCB in the CU.

I know this is a few months old but I found it quite an interesting thread.

Presumably each switch is on a spur and doesn't that mean the supply side of each switch module will have 2xL and 2xN cores - continuing the ring in and out.

So there could be more than 13a flowing through the terminals on the supply side of the switch. But maybe that's ok because its not flowing through the switch ?

EDIT CORRECTION: Apologies I corrected this a couple of times !

 

[Midwest]

Don’t think so. Each DP switch will have feed in and a feed out, for each live & neutral terminal, thus completing the RFC. The load side of the switch will feed the appliance.

 

[Deleted member 112675]

Don’t think so. Each DP switch will have feed in and a feed out, for each live & neutral terminal, thus completing the RFC. The load side of the switch will feed the appliance.

Thanks for that but the feed in surely has to continue the ring thus would need two cores per terminal? (And the load side as you say off to the socket). So each DP switch controls a single spur with a single socket.

 

[Midwest]

Thanks for that but the feed in surely has to continue the ring thus would need two cores per terminal? (And the load side as you say off to the socket). So each DP switch controls a single spur with a single socket.

Yep, as said feed in and feed out per terminal. And each DP switch feeds an appliance via a single cable. We at cross purposes?

 

[Deleted member 112675]

Yep, as said feed in and feed out per terminal. And each DP switch feeds an appliance via a single cable. We at cross purposes?

Sorry yes might be my fault - I had to change my original post.
So if there is feed in and feed out that could easily go beyond 13a (and possibly beyond 20a depending on where it is in the ring?). Not sure if its an issue in practice though?

ANd in any case no more than 13a would be going through the switch.

 

[Strima]

So if there is feed in and feed out that could easily go beyond 13a (and possibly beyond 20a depending on where it is in the ring?).

Both cables will be in the same terminal and clamped together so the current is flowing through the cables and not the terminal block.

Most switches would normally feed a single socket for an appliance however you could potentially use a double socket-outlet for two appliances, these are normally rated for a maximum of 20 amps hence the 20 amp rating on the switches.

This is down to diversity of the loads and unless you have two three bar heaters plugged in it's doubtful you'll ever exceed 20 amps on a switch.

 

[telectrix]

might never exceed 20A through the switch but why do i keep finding them burnt out and occasionally bridged? spawn of the devil, would never fit myself.

 

[Taylortwocities]

might never exceed 20A through the switch but why do i keep finding them burnt out and occasionally bridged? spawn of the devil, would never fit myself.

They burn out because they’ve been installed badly, often by fitters of kitchens. And I agree, pain in the whotsit, and mostly pointless.

 

[Midwest]

might never exceed 20A through the switch but why do i keep finding them burnt out and occasionally bridged? spawn of the devil, would never fit myself.

There's thousands out their with no problems. Perhaps those that have caused issues are like problems RFC's have, to the unanitiaive. (cant be arsed to spell it correctly)

 

[Oliver Luke Martin]

Sorry all to revive an old post. I’ve often thought 20A switches are underrated so was looking for alternative. I’m installing a grid switch to feed appliances in the customers house. I’m using Scolmore Minigrid 32A switches, this allows full current through the RFC without worrying about terminals over heating. What are your views on this solution?

 

[Taylortwocities]

Don’t see the problem. A grid switch isolating a single appliance should never have to handle more than 13A. Unless you have a double socket, even then the expected load shouldn’t exceed 20A.

. I’m using Scolmore Minigrid 32A switches, this allows full current through the RFC

But you should never be interrupting the ring final. That’s the only place where the higher current might be found. Even then it’ll be around 20A max.
The switch will only ever see the current on the (switched) spur.

 

[Oliver Luke Martin]

Incase I wasn’t clear the 32A switch allows the RFC to continue at a potential full load on the supply as the terminals are rated at 32A as a pose to 20A on many grid switches. Only one socket/appliance on the load side.

 

[Strima]

Incase I wasn’t clear the 32A switch allows the RFC to continue at a potential full load on the supply as the terminals are rated at 32A as a pose to 20A on many grid switches. Only one socket/appliance on the load side.

The switch is rated at 20 amps not the supply side terminals which are designed for much higher currents.

 

[ackbarthestar]

I thought a 20A DP switch was rated at 20A. Please point me in the directions of the specifications for a 20A DP switch where the input terminals are something else

 

[Avo Mk8]

Looking at the specifications of MK 20A grid switches, they quote the input terminal capacity as 3 x 2.5mm or 2 x 4mm or 1 x 6mm. The MK 32A cooker switch has the same input terminal capacity figures. I don't see any MK specification for the maximum through current between two conductors stuffed in the same input terminal, but I suspect from the above is that it would be more that the rating of either of those switches.

 

[Dartlec]

Sorry all to revive an old post. I’ve often thought 20A switches are underrated so was looking for alternative. I’m installing a grid switch to feed appliances in the customers house. I’m using Scolmore Minigrid 32A switches, this allows full current through the RFC without worrying about terminals over heating. What are your views on this solution?

I suspect the terminals on any switch are probably not the limiting factor, it's the connections within the switch that may be. I see their 32A switches are double width, so the extra space may allow for meatier internals and a higher rating on the throughput. They also do a 20A DP switch of the same size though, so maybe it's mostly marketing. Would be interesting to take one of each apart and see what the difference is.

The terminals on all but the cheapest accessories could probably easily cope with 32A for straight in and out connections. Sockets are mostly rated at 20A after all but still are fine on a ring - whenever I've seen overheat damage it's usually due to loose connections, and it's the plastic that gives up not the terminal.

Having said that, I have started using the minigid light switches a lot recently, and they seem pretty well made so seems like a neat solution to provide isolation of under worktop appliances.

 

[Strima]

I thought a 20A DP switch was rated at 20A. Please point me in the directions of the specifications for a 20A DP switch where the input terminals are something else

TBH I could have worded my response a little better. The supply side terminals don't have to be load rated as such due to the fact they clamp conductors together and take no load themselves.

 

[ackbarthestar]

Ok, forget about diversity and unlikely loading conditions for the moment.

A grid switch assembly with 3 DP switches each supplying 3kW loads.

Now if the Grid switches are at the centre of the ring then we would expect 37.5A available when all the loads are active and for a DC supply 18.5A divided between the outer two switches supplying 12.5A to the middle switch but as its ac current the current flow across all of the input terminals would 37.5A, 25A, 12.5A every 10ms then the same in the opposite direction for a further 10ms continuing until the loads are switched off.

If the grid switch assemby is ofset in the ring then the current flow through the switches would be proportionally more on one side of the ring.

The input terminals are therefore loaded by the loads throughout the ring.
very important to make sure the terminals are tight.

 

[Ian1981]

clamp the line at the meter tails. I’ve had all my appliances on, drier, washer, dishwasher, never got over 20 amps, all be it, it was for around half an hour only, you’ll never see 30+ amps at all unless they are a resistive load like a water heater

 

[Taylortwocities]

you’ll never see 30+ amps at all unless they are a resistive load like a water heater

Which shouldn’t be connected to a ring final in the first place!

 

[Pretty Mouth]

A grid switch assembly with 3 DP switches each supplying 3kW loads.

Now if the Grid switches are at the centre of the ring then we would expect 37.5A available when all the loads are active and for a DC supply 18.5A divided between the outer two switches supplying 12.5A to the middle switch but as its ac current the current flow across all of the input terminals would 37.5A, 25A, 12.5A every 10ms then the same in the opposite direction for a further 10ms continuing until the loads are switched off.

I'm not sure that's right fella, unless there's something I'm missing?

Each DP switch would have its equipment's load of 12.5A flowing through it, both line and neutral.
The terminals of the switch dead centre of the ring would only have that 12.5A passing through them.
The one to the left would have it's load of 12.5A passing through its terminals, plus half of the centre switch's load as it passes by on its way to/from the consumer unit. So 12.5+6.25=18.75A. Exactly the same for the switch to the right.

The fact that it's AC is irrelevant.

 

[ChrisElectrical88]

Looking at the specifications of MK 20A grid switches, they quote the input terminal capacity as 3 x 2.5mm or 2 x 4mm or 1 x 6mm. The MK 32A cooker switch has the same input terminal capacity figures. I don't see any MK specification for the maximum through current between two conductors stuffed in the same input terminal, but I suspect from the above is that it would be more that the rating of either of those switches.

There’s not a chuffing chance you can get 2x4mm or 3x2.5 in a terminal on a 20A MK grid switch. There no where near a cooker switch size. Got to be a mistake in the literature there.

I said it in April 2019, they may be compliant, you need a 47mm deep back box, but every single one of these I have seen has been as rough as a badgers (thank you whoever pointed out a badgers arse is tougher than a bears arse recently) arse. More copper showing behind these things than the 1.5mm line in the Doncaster cable factory.

 

[Ian1981]

A 4 gang grid is specced on our new build sites ,47 mm double back box, Deta grid modules, no issue whatsoever in connecting them in a proper manner, in the 5 years I’ve been there, there’s never been a single issue that I know of regarding installing a grid system on a kitchen ring final.
That’s washer, dishwasher, fridge and hob ignite or a combination of cooker hood as sometimes a 2 gang grid is also fitted for the utility accessories (washer and drier)