How to understand power ratings and interaction with fuses

[Ellsa100]

Hi there

I have a few questions related to the power ratings of certain devices and how this interplays with the fuse. The reason I am asking this is whether I can safely add new plug sockets off an existing 3A fused spur (I plan to only use these plug sockets for low power devices).

The current 3A fused spur is connected to a Heatmiser wiring centre which controls our wet underfloor heating system. The wiring diagram of the Heatmiser wiring centre is below and my queries relate to the yellow highlighted sections.

1. Given Power (P) = Current (I) x Volt (V), if the power consumption is 7W and the supply voltage is 230V, wouldn't that mean the current the unit consumes is 0.03A (i.e. 7/230) or, if not, please can someone explain?

2. If I am correct in concluding the current consumed is 0.03A, why is there (a) a 3A mains supply fuse (this is in a 3A fused spur) and (b) a 5A fuse on the board? How much 'excess' fuse capacity is on the 3A fuse if the Heatmiser wiring centre is the only device connected to it?

3. If I am correct in concluding the current consumed by the Heatmiser wiring centre is 0.03A, how can the max load on the different boiler/H/W/zone outputs be 3A 230V with a max total load of 5A?

4. Would it be safe to run a couple of plug sockets coming off the 3A fused spur (I would want to use low power devices)? If so, how much spare "ampage" should there be for those plug sockets (would it, for example, be 3A less 0.03A)?

Thank you in advance

 

[davesparks]

No, don't add sockets to the load side of the isolator for your heating.

There's many reasons for this including that any fault on anything plugged into the sockets will leave you without heating.
Anything plugged in which overloads the 3A fuse will leave you without heating.

A 3A fuse is installed because it is the nearest standard sized fuse to the load current of the system, and it is adequate to protect the cables used.

A 3A fuse also allows for any inrush currents in the system.

 

[Ellsa100]

Thanks.

I think the current 3A fuses spur is off the ring main.

Could I split the spurs into two fused spurs, each of 3A? So one of the 3A fused spurs connects to Heatmiser, the other 3A fused spur connects to some plug sockets? Would I need the total power of all plugs connected to a 3A fused plug socket to be less than 230 x 3 = 690 watts (ie volt x current)?

Thanks again

 

[Lucien Nunes]

1. That calculation is correct for a resistive load but does not take into account power factor of inductive or capacitive loads. Some devices, e.g. the pump, will take more current that P/V would suggest, up to 50% more in some cases.

2. The Heatmiser control centre itself only uses 7W, but it feeds other devices like pump(s) that consume more. The 5A fuse protects the tracks on the PCB in the Heatmiser against burnout if something like the pump short-circuits, and some fool has put a 13A fuse in the fused connection unit (i.e. spur box, FCU.) An advantage of using a 3A fuse in the FCU is that this is likely to fail first in the event of a fault vs. the internal 5A, and it is easier to replace than the internal one.

3. The individual outputs are controlled by relays each of which has a maximum rating of 3A. This is enough for the entire system but as it happens the internal wiring of the control centre is physically capable of carrying 5A. It has a safety margin when protected by the recommended 3A fuse in the spur. I agree it is a little inconsistent to recommend the 3A fuse for a device capable of handling 5A, but note the last part of answer 2 above.

4. As per @davesparks above, the FCU is required as a means of isolation for the heating system and should not be interfered with. Whether it is permissible to add another spur to feed socket outlets will depend on the configuration of the circuit, whether the FCU is a point in the ring or a spur from it. It would be non-standard to limit a socket-outlet to 3A and probably unnecessary - a 13A fused spur would be more satisfactory.

 

[Ellsa100]

1. That calculation is correct for a resistive load but does not take into account power factor of inductive or capacitive loads. Some devices, e.g. the pump, will take more current that P/V would suggest, up to 50% more in some cases.

2. The Heatmiser control centre itself only uses 7W, but it feeds other devices like pump(s) that consume more. The 5A fuse protects the tracks on the PCB in the Heatmiser against burnout if something like the pump short-circuits, and some fool has put a 13A fuse in the fused connection unit (i.e. spur box, FCU.) An advantage of using a 3A fuse in the FCU is that this is likely to fail first in the event of a fault vs. the internal 5A, and it is easier to replace than the internal one.

3. The individual outputs are controlled by relays each of which has a maximum rating of 3A. This is enough for the entire system but as it happens the internal wiring of the control centre is physically capable of carrying 5A. It has a safety margin when protected by the recommended 3A fuse in the spur. I agree it is a little inconsistent to recommend the 3A fuse for a device capable of handling 5A, but note the last part of answer 2 above.

4. As per @davesparks above, the FCU is required as a means of isolation for the heating system and should not be interfered with. Whether it is permissible to add another spur to feed socket outlets will depend on the configuration of the circuit, whether the FCU is a point in the ring or a spur from it. It would be non-standard to limit a socket-outlet to 3A and probably unnecessary - a 13A fused spur would be more satisfactory.

Thanks, this is super helpful and makes sense.