Grid switches on 32A ring

[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)