by ericmark »
Tue Aug 19, 2014 11:42 am
The old Wylex box had four different devices which could be used. At first it was a re-wirable fuse using standard fuse wire, then the cartridge fuse holder replaced these I think it needed base swapping and then a push button MCB and finally a switch type MCB the WYLEX 40A SP Type B Curve MCB Product Code: 77992 is still made "screwfix number" the fuses in screwfix are only up to 35A can't see any fuse holders sold.
In theroy before swapping from fuse to MCB you should test the earth loop impedance the MCB is in fact two devices in one the thermal trip is slow acting and the magnetic trip is fast acting but needs 5 times the current of the thermal trip and that's why measuring earth loop impedance is important. Unlike a fuse where if slightly over it takes slightly longer with the MCB slightly over takes a lot longer to trip.
Showers come in two major types the power shower uses very little power and likely would need a 6A MCB but the instant heat type uses a lot more current and either a 32A or 40A MCB.
There are 3 cable sizes which could be used 4mm sq, 6mm sq, and 10mm sq some times you can see size written on the cable but hard to work out from just looking at the cable. Also cable current carrying capacity is also controlled by how it is fitted and fitting a 40A MCB with 4mm cable could allow the cable to overheat.
Reading the size of the shower you may know how much it needs but often showers have fuses or MCB's below the size they really need with the idea we do not use them for long so fitting a 40A MCB without knowing what size the cable is could cause overheating will prolonged use or fault.
Since the fuse is missing fault is a very real possibility.
With the idea it could be a DIY fit also possible the RCD protection is missing.
Assuming a RCD is fitted and it is an instant heat type fitting a 32A MCB is unlikely to cause a problem but at £10 each I would say getting it tested first may be the better idea.
If you do change to a MCB yourself be aware when the base screw is removed it exposes live parts so main isolator needs to be off and also the fuse cover will no longer fit.
I strongly recommend you get an electrician to do this for you and test it before trying to fit the MCB. Likely the fuse was removed because there is a fault and just replacing could be dangerous.
