Power Shower power.

[ericmark]

They are not if they are suggesting a FCU on the lighting circuit, though that is not clear. FCUs are not required or needed on lighting circuits. I doubt if the cable on the transformer is so small it requires a 3 amp fuse (it would not be compliant if it was). The fuse is to protect the cable not the transformer. Any way a 3a fuse has no discrimination against a 6a MCB used for lighting circuits.

With a portable appliance you are correct the fuse protects the cable, but with fixed appliances the manufacturer can require the device to protect the appliance to be remote from the appliance, often in a more accessible location. And it is common for transformers to have a simple soldered spring which will release on over heating which is rather slow acting, and bathroom fans can fail when stalled as the windings are just not man enough to take extra current and they will not take 3A which is one of the preferred two fuse sizes.

To fit a 3A fuse to a supply from a B6 MCB I will agree seems pointless, as 9 times out of 10 the MCB will trip first, bulbs should have a built in fuse, as most BA22d lamp holders only rated at 2A but lighting MCB normally 6A, and when houses had fuse boxes it was very rare for the 6A fuse to rupture when a lamp on blowing ionised causing the well known flash as it failed, however is was a regular problem with MCB's that they would trip. However it was the magnetic part of the trip which operated, and the over load for a motor is to stop the supply should the motor stall so it would be the thermal part of the trip which would save a motor so to protect a motor there could well be a fuse required.

I don't think a BS1362 fuse would be fast enough to protect the transformer, it would likely need a special fuse to do that of a very precise rating which would not fit in a standard FCU however to say with fixed appliance the fuse only protects the cable is wrong.

Personally I would look at the motor rating, if powering from lights, and would be considering how small of a fuse I could fit, the problem powering anything from lights is with a fault you not only have the fault but are also plunged into darkness, it is not an electrical problem, it is simple risk assessment, if the pump gets water into it, and it trips the RCD will the person in the shower or anyone else in the house be in danger because they can't see? In my old house with stairs in centre of house, I fitted an emergency light over the stairs. This house has a street light outside and a window so risk is very small.

But until plumbing sorted really does not matter, I found needed 22 mm pipes to header tank to ensure the shower pump would draw air rather than empty the cistern with immersion heater in it. That is the main concern with power shower, it often can't be supplied with existing hot and cold pipes as it could suck the cistern dry. 22 mm does not get air locks in the main, 15 mm may get an air lock, and 10 mm plastic very likely to get air lock, so it is down to pipe sizes between header tank and cistern. I am no plumber, but I did see the risk and I did turn off the mains and test to see with mains failure would the cistern be drained, and I had to move the hot water supply pipe to ensure it did not happen.

 

[winston1]

I don't think a BS1362 fuse would be fast enough to protect the transformer, it would likely need a special fuse to do that of a very precise rating which would not fit in a standard FCU

So once again manufacturers instructions are wrong!

 

[winston1]

But until plumbing sorted really does not matter, I found needed 22 mm pipes to header tank to ensure the shower pump would draw air rather than empty the cistern with immersion heater in it. That is the main concern with power shower, it often can't be supplied with existing hot and cold pipes as it could suck the cistern dry. 22 mm does not get air locks in the main, 15 mm may get an air lock, and 10 mm plastic very likely to get air lock, so it is down to pipe sizes between header tank and cistern. I am no plumber, but I did see the risk and I did turn off the mains and test to see with mains failure would the cistern be drained, and I had to move the hot water supply pipe to ensure it did not happen.

The immersion heater is normally in the copper cylinder not the cistern. The outlet is at the top of the cylinder and once the water level falls below this point it is impossible to suck any more out..

 

[ban-all-sheds]

They are not if they are suggesting a FCU on the lighting circuit, though that is not clear. FCUs are not required or needed on lighting circuits.

But using them is not a contravention.