Electric Panel Heater on radial circuit

[JohnW2]

If a circuit is electrically compliant with a 30A BS3036 fuse then this could be replaced by a 40A MCB.

I don't think it's as simple as you imply ...

I think you're probably overlooking the fact that the 'design current' (essentially the 'maximum load'), Ib, of a circuit is effectively defined by the ';rating' (in) of the OPD, regardless of the nature of that OPD - in particular that if one increases the rating of the OPD from 30A to 40A then, regardless of the nature of the IOPDs, one will have increased the design current (hence maximum load) from 30A to 40A.

The CCC of a cable, Iz, is obviously the same for any given installation method, regardless of the nature or rating of the OPD. The 'worst case' (lowest CCC) allowed by the regs for a ring final circuit is 20A.

Given that 'lowest permissible CCC of 20A' ...

If the rating (In) of the OPD is 30A then the design current (maximum load) (Ib) is also 30A, regardless of the nature of the OPD. Since that Ib is appreciably less than double the Iz (of 20A), the designer may be satisfied that,even when the circuit is loaded to its full Ib, it is 'not likely' that any part of the cable of the ring circuit will be overloaded (as required by reg).​

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However, if the In of the OPD were increased from 30A to 40A (regardless of the nature of the OPDs), that would be exactly double the Iz (20A) and hence, if the circuit were loaded to its full Ib, it would be inevitable that part of the cable of the circuit would be overloaded unless the load were applied to the ring in a perfectly balanced fashion, so it could not be said that such an event was "not likely".​

 

[martygturner]

That is just as dependent on the number of radial circuits, as it is on the number of ring circuits in the premises.

Here I have two rings, and around five radial socket circuits. Both methods have their good points.

Sorry I don't get that, you have a ring fed from 1 rcbo then split the ring to form two independent circuits fed from two rcbo's... if the ring develops a fault then that's that power off, if a radial develops a fault you have the other radial. By splitting a ring you gain an extra circuit so its not dependant on the number of circuits as you have taken one and increased it to two....

 

[ebee]

yes the good thing about RCBOs over single or dual RCDs (RCCBs) is that only affecting one circuit rather than two or more in an earth fault situation, I agree. Whether ring or radial I am happier if with have a choice of at least two circuits in some locations (Kitchens probably top of that list in most properties) but I am also happy with rings and/or radials, both having advantages and disadvantages over the other.
In an ideal world, which does not exist, then if we can move towards two circuits minimum of each kind for important places (such as kitchens, living rooms and stairways both for lighting and power use than we shall have a bit more redundancy.

 

[Harry Bloomfield]

Sorry I don't get that, you have a ring fed from 1 rcbo then split the ring to form two independent circuits fed from two rcbo's... if the ring develops a fault then that's that power off, if a radial develops a fault you have the other radial. By splitting a ring you gain an extra circuit so its not dependant on the number of circuits as you have taken one and increased it to two....

You cannot just magically split the ring in two, and double the number of circuits - most consumer units lack the spare capacity to add more MCB's.

I do appreciate the value of having more circuits, in fact not many weeks ago I swapped my consumer unit, to one with double the number of ways, so I coud do exactly that.

 

[martygturner]

You cannot just magically split the ring in two, and double the number of circuits - most consumer units lack the spare capacity to add more MCB's.

You can take out the RCD's and replace them with RCBO's - RCD takes up two RCBO spaces + the mcb space which now gives three spaces when you had one before. Given that the box is suitable.

 

[Harry Bloomfield]

You can take out the RCD's and replace them with RCBO's - RCD takes up two RCBO spaces + the mcb space which now gives three spaces one you had one before.

Again, you are making many assumptions to create the essential space. Back in the real world, many installs may not have spare locations in the consumer unit, may not have RCD's, and the CU's age may mean MCB's are no longer available. That was the position I personally was in.