Two high-power appliances on a single 40A RCD

Okay. Bravado grandstanding and hysterical outbursts seem to be the order of the times; a deflection away from an initial outrageous statement, taking the narrative to a wholly unrelated tangent with equally bombastic claims. I suppose it clouds the issues sufficiently for the message spouted by the most loud and most brash to be the headline for the casual observer.


The simple issue was an existing circuit. A shower circuit. It had already been installed, presumably fit for purpose. The Ib was assessed when it was installed diversity applied, that formed the type/rating of OCPD and the cable. So In>=Ib.

Someone wants to use that circuit for something else on the basis that the shower is unlikely to be used.


Salami diversity should not be applied to a circuit that has already had diversity applied to it.

has anybody mentioned the D word?


Put another way, you have two 40 A showers, and a 40 A cooker on a 100 A supply. Does that break the rules?

 

The above is diversity applied to the whole installation and is perfectly normal. It is a whole different thing to again apply diversity  to one of the above shower circuits, again, in order to justify adding another load to it. That is Salami Style.


We are in the realms of BS7671 but some are using methods outside this remit, specifically ESCQR, in an attempt to underpin some dubious benefit of Badger work in a BS7671 context.

 

By example take the cables in your street. They are probably protected by a 600A fuse and feed many houses. You may have a 10mm2 service if you have an old house (I used to some years back). A short on the service cable can be quite spectacular because the short circuit rating is quite suspect at a 5kA fault. If each house takes power and it exceeds about 1kA for a reasonable period the 600A fuse will blow, but it takes time. The street cable will not really be warm. If they all have off peak heating and 800A flows all night (not unusal) the cable will be pretty hot by morning. Is our little used shower a problem, very unlikely.

 

The point is the vast majority of this DNO equipment is not inside the house. Differing application, differing risk and very much different methods to control the risk.


Do not confuse risk with only  being something that results in danger. With respect to efficacy, that needs to be also evaluated. Again the circuit was originally installed to have a realistic lifespan surely? Exceeding the In of an MCB multiple times is eventually going to result in its failure and it needs to be replaced. Fit for its intended purpose for a reasonable period of time is the usual expectation.


Conflating the objections to simultaneous usage as charges it is " dangerous" practice is misleading. I expect one form of extremism generates the equal an opposite extreme response.


The OP said

Can anyone advise on a safe and legal way to do this, ensuring that only one of the two appliances can be connected at any one time?

 

 

Some simple solutions wrt BS7671 were given.


Then someone who should know better, actually states

 

What is wrong with simultaneous use, it will trip the MCB at some point, strangely that is what the MCB is for!!!!

 

It is a circuit breaker, not a load limiter.

If, purely just for example, the shower was 11kW with a 32 amp OCPD, then Ib >In. Would that be a fit for purpose arrangement? Would it be fit and proper to use the MCB as a load limiter, restricting shower use to a few minutes before it trips?


The some undeserving electrician has the temerity to say, without genuflecting first

The screwfix forum says
433.1.1 [i]   the rated current of the protective device is not less than the design current.


It is one thing to say something is not going to be used. It is still there and could be used. The kitchen fitter is being responsible; why should they take on additional risk?

The answer by Broadgage concerning "Shower priority unit" is a legitimate way to go. For legitimate, read sensible.

Circuit breaker as a "load limiter" is a bit rough; for the cavalier it may be a badge of honour , but really should have no place for installation work done for reward and done to some kind of standard.

Some want a "trial by EICR code".  If Ib is demonstrably > In,  it is then recommend improvement.  You start with a shower circuit where In is equal to or just below Ib,  then you add a double oven, then suddenly Ib > In.


You are going to engage in installation works; adding to a circuit. The expectation is that, at a minimum, you are not going to leave things  worse than when you stated. Creating a "recommend improvement" out of a situation that was originally compliant with BS7671 really is egg on the face territory.

 

Say I have a large motor circuit say 100 kW which is designed to start a few times per hour. I use a type D (MCCB but this is to help you) CPD. Do the cables need to be rated at 6 times the running current? If so which regulation are you quoting? Just some background this motor may take a minute (60 seconds) to reach FLC due to the load inertia. What size cables should I use? How do I control the number of starts per hour? How is this different to the example I gave or the OP?

"How is this different to the example I gave or the OP?"  Answer:  You are not going to add a double oven to it later on.


David, your discussions on diversity and how it should be applied by Application have merit, but not in context of this OP. Some right words, but the wrong discussion.  That said though, the general thrust seems to be a attempt at besmirching the integrity of professional electricians. That is not welcome and hints at an ivory tower mentality.

 

 

Okay. Bravado grandstanding and hysterical outbursts seem to be the order of the times; a deflection away from an initial outrageous statement, taking the narrative to a wholly unrelated tangent with equally bombastic claims. I suppose it clouds the issues sufficiently for the message spouted by the most loud and most brash to be the headline for the casual observer.


The simple issue was an existing circuit. A shower circuit. It had already been installed, presumably fit for purpose. The Ib was assessed when it was installed diversity applied, that formed the type/rating of OCPD and the cable. So In>=Ib.

Someone wants to use that circuit for something else on the basis that the shower is unlikely to be used.


Salami diversity should not be applied to a circuit that has already had diversity applied to it.

has anybody mentioned the D word?


Put another way, you have two 40 A showers, and a 40 A cooker on a 100 A supply. Does that break the rules?

 

The above is diversity applied to the whole installation and is perfectly normal. It is a whole different thing to again apply diversity  to one of the above shower circuits, again, in order to justify adding another load to it. That is Salami Style.


We are in the realms of BS7671 but some are using methods outside this remit, specifically ESCQR, in an attempt to underpin some dubious benefit of Badger work in a BS7671 context.

 

By example take the cables in your street. They are probably protected by a 600A fuse and feed many houses. You may have a 10mm2 service if you have an old house (I used to some years back). A short on the service cable can be quite spectacular because the short circuit rating is quite suspect at a 5kA fault. If each house takes power and it exceeds about 1kA for a reasonable period the 600A fuse will blow, but it takes time. The street cable will not really be warm. If they all have off peak heating and 800A flows all night (not unusal) the cable will be pretty hot by morning. Is our little used shower a problem, very unlikely.

 

The point is the vast majority of this DNO equipment is not inside the house. Differing application, differing risk and very much different methods to control the risk.


Do not confuse risk with only  being something that results in danger. With respect to efficacy, that needs to be also evaluated. Again the circuit was originally installed to have a realistic lifespan surely? Exceeding the In of an MCB multiple times is eventually going to result in its failure and it needs to be replaced. Fit for its intended purpose for a reasonable period of time is the usual expectation.


Conflating the objections to simultaneous usage as charges it is " dangerous" practice is misleading. I expect one form of extremism generates the equal an opposite extreme response.


The OP said

Can anyone advise on a safe and legal way to do this, ensuring that only one of the two appliances can be connected at any one time?

 

 

Some simple solutions wrt BS7671 were given.


Then someone who should know better, actually states

 

What is wrong with simultaneous use, it will trip the MCB at some point, strangely that is what the MCB is for!!!!

 

It is a circuit breaker, not a load limiter.

If, purely just for example, the shower was 11kW with a 32 amp OCPD, then Ib >In. Would that be a fit for purpose arrangement? Would it be fit and proper to use the MCB as a load limiter, restricting shower use to a few minutes before it trips?


The some undeserving electrician has the temerity to say, without genuflecting first

The screwfix forum says
433.1.1 [i]   the rated current of the protective device is not less than the design current.


It is one thing to say something is not going to be used. It is still there and could be used. The kitchen fitter is being responsible; why should they take on additional risk?

The answer by Broadgage concerning "Shower priority unit" is a legitimate way to go. For legitimate, read sensible.

Circuit breaker as a "load limiter" is a bit rough; for the cavalier it may be a badge of honour , but really should have no place for installation work done for reward and done to some kind of standard.

Some want a "trial by EICR code".  If Ib is demonstrably > In,  it is then recommend improvement.  You start with a shower circuit where In is equal to or just below Ib,  then you add a double oven, then suddenly Ib > In.


You are going to engage in installation works; adding to a circuit. The expectation is that, at a minimum, you are not going to leave things  worse than when you stated. Creating a "recommend improvement" out of a situation that was originally compliant with BS7671 really is egg on the face territory.

 

Say I have a large motor circuit say 100 kW which is designed to start a few times per hour. I use a type D (MCCB but this is to help you) CPD. Do the cables need to be rated at 6 times the running current? If so which regulation are you quoting? Just some background this motor may take a minute (60 seconds) to reach FLC due to the load inertia. What size cables should I use? How do I control the number of starts per hour? How is this different to the example I gave or the OP?

"How is this different to the example I gave or the OP?"  Answer:  You are not going to add a double oven to it later on.


David, your discussions on diversity and how it should be applied by Application have merit, but not in context of this OP. Some right words, but the wrong discussion.  That said though, the general thrust seems to be a attempt at besmirching the integrity of professional electricians. That is not welcome and hints at an ivory tower mentality.