Mixing ring mains in the kitchen

[PrenticeBoyofDerry]

I prefer the cooker hood on the cooker circuit.

And I don't, but as you say personal choice.
I prefer to still be able to extract, even if the cooker circuit is off.

 

[riveralt]

Well for starters, grid switches are typically only rated to 20A

So are sockets.

Where does it say that?
The MK sockets I use say 13A per outlet, except for three gang which is 13A in total.

 

[ban-all-sheds]

BS 1363

 

[riveralt]

That is the minimum standard to reach BS1363 - nothing to manufacturers exceeding that is there?

 

[ban-all-sheds]

No, but you can never rely on sockets being rated at anything more than 20A, so it's a bit illogical to say that's OK on a ring final but other 20A-rated accessories are not.

 

[riveralt]

No, but you can never rely on sockets being rated at anything more than 20A, so it's a bit illogical to say that's OK on a ring final but other 20A-rated accessories are not.

But following this approach then putting a washing machine and tumble dryer on the same double gang socket and switching them on at the same time - is wrong?
I disagree and a blanket ban of having two 13A rated devices on the same double gang socket is illogical.
The 20A standard for sockets is set for continuous use with a minimum of 4hrs and maximum of 8hrs and with a maximum rise in temperature - I cannot remember the exact number but it must be close to the 70° temperature of the cable.
In the long distant past I'm sure I read that the likes of MK probably meet the target 20A target indefinitely, and only with continuous exposure to higher rates for very long periods of time, certainly more than 8hrs do problems of heat degradation start to occur.
But there aren't that many domestic situations (apart from a cannabis farm) that you will have a continuous 26A load. Even the washing machine/tumble dryer example given above will rarely, if ever, breach the 20A standard.

 

[flameport]

As for the grid switches being unsuitable for connection into the ring mains, from the research I've done, it would be no different to looping through the cable from a 13A socket in a ring - these are rated to 20A - the 2.5mm2 cable is rated to 20A - I'm using the switch as part of the ring main loop - not to switch or break the ring main.

The problem with an arrangement of grid switches is that it concentrates all of the load into a single point - the very thing that a ring circuit is not designed for.
It also introduces unnecessary extra fuses, additional points of failure and increases the cost & complexity of installation.

I'd like a more obvious way to isolate these appliances in an emergency.

You already have this - it's the big red switch in the consumer unit.

For maintenance of a single appliance, you unplug it from the socket outlet.

 

[JohnW2]

The problem with an arrangement of grid switches is that it concentrates all of the load into a single point - the very thing that a ring circuit is not designed for.

Yes, but the nature of kitchen rings is that such a bank of grid switches is quite likely to be near the mid-point of the ring - and having all loads concentrated at (or near to) the centre point is really the best possible situation for a ring circuit, isn't it?

Kind Regards, John

 

[ban-all-sheds]

But following this approach then putting a washing machine and tumble dryer on the same double gang socket and switching them on at the same time - is wrong?

Could be.

I disagree and a blanket ban of having two 13A rated devices on the same double gang socket is illogical.

Maybe it is, but ignoring what the standard requires and refusing to accept it because it is illogical is worse than illogical itself - it is mad.

The 20A standard for sockets is set for continuous use with a minimum of 4hrs and maximum of 8hrs and with a maximum rise in temperature - I cannot remember the exact number but it must be close to the 70° temperature of the cable.

Be that as it may....

In the long distant past I'm sure I read that the likes of MK probably meet the target 20A target indefinitely, and only with continuous exposure to higher rates for very long periods of time, certainly more than 8hrs do problems of heat degradation start to occur.

Quite possibly.

And as you observe, there's nothing to stop Acme Electrical making a DSO which can handle 26A indefinitely, and still keep your beer cold. But from a design POV you may not assume that a generic DSO can handle more than 20A. Which is the point I was making in relation to the 20A rating of grid accessories.

But there aren't that many domestic situations (apart from a cannabis farm) that you will have a continuous 26A load. Even the washing machine/tumble dryer example given above will rarely, if ever, breach the 20A standard.

And another thing you may not do is to interpolate a time for which a generic DSO will cope with 26A.

 

[JohnW2]

In the long distant past I'm sure I read that the likes of MK probably meet the target 20A target indefinitely, and only with continuous exposure to higher rates for very long periods of time, certainly more than 8hrs do problems of heat degradation start to occur.

You may be recalling the following, allegedly from some MK catalogue (I've never found the original) which I found quoted in the IET forum in late 2009 and have posted here a few times...

All MK socket-outlets are manufactured to comply with BS1363 part 2: 1995 and are rated at 13A per unit. Double socket-outlets have been manufactured and tested to exceed this rating by margin that allows electrical safety and reduces the risk of heat and mechanical damage to components due to overloading. It should be noted that BS1363 part 2: 1995 does not allow double sockets to operate at twice the permissible maximum loading and it should be remembered that double socket-outlets are not manufactured to be able to withstand a 26A load for sustained periods of time.

Research by ourselves and third party organisations has shown that all MK double sockets can safely withstand a continuous load of 19.5A for an indefinite period. Increasing the load slightly will begin to cause heat and mechanical stresses on the components in a relatively short period. Testing showed that a load of 22.3A was sufficient to cause heat stress that would cause a browning of the faceplates and sufficient heat to cause insulation damage to cable cores. A load of 24A for 43 hours was sufficient to cause significant heat damage to the material in which the socket-outlet was situated and within 75 hours sufficient to cause significant damage that would lead to the very real potential of fire.

MK recommend that users of their sockets consult professional design Engineers when designing installations to avoid the possibility of heat and mechanical stress to components and installations caused by overloading of MK socket-outlets.

This is more-or-less consistent with what you said (19.5A 'indefinitely'), although it is not clear what 'relatively short period' will result in 'heat and mechanical stresses' if current increases to 'slightly more' than 19.5A.

I agree that one would not really expect that 26A would often be drawn for long enough, in domestic situations, to do any harm - but we have seen a good few reports (some with pics) of what may have happened because of running two 'high current loads' off one double socket. I have to say that, particularly given that such issues are most likely to arise in 'aesthetically less important' places (like utility rooms), I tend to use single sockets on a "why not?" basis.

Kind Regards, John

 

[ban-all-sheds]

It should be noted that BS1363 part 2: 1995 does not allow double sockets to operate at twice the permissible maximum loading

That's like saying that legislation requiring the outside of a kettle to get to no more than x°C when used to boil a full load makes it illegal for it to get to less.

 

[JohnW2]

It should be noted that BS1363 part 2: 1995 does not allow double sockets to operate at twice the permissible maximum loading

That's like saying that legislation requiring the outside of a kettle to get to no more than x°C when used to boil a full load makes it illegal for it to get to less.

That doesn't seem a valid analogy, but I do agree that MK's wording was very poor. Presumably what they meant to say (as you yourself have recently written) was that double sockets manufactured to comply with BS1363 could not be guaranteed to be able to safely operate at 26A total load.

Kind Regards, John