I have a 8.5kw electric shower which has stopped working, looking in the fuse box the Wylex (push in type, not mcb) carrier and fuse are burnt!
The shower was fitted many years ago by Southern Electric, so I can only assume the cable is correct. I replaced the actual shower unit with a new one, pratically identical one from Wickes/Triton.
Should I be concerned about this?. I can easily replace the fuse and hopefully catridge, but it has scortched some of the bit the fuse plugs into - can this be replaced.
Yes. Such overheating should not occur and if not attended to could become a fire risk. Possible causes:
- loose terminal screw on live wire coming out of top of fuseway,
- loose screw within fuse carrier (if re-wireable type),
- circuit wired with grossly undersized cable,
- loose fuse contact spring in consumer unit,
- 30 A (red) fuse installed (8.5 kW shower needs a 45 A (green) fuse).
Yes, the "scorched bit that the fuse plugs into" is just a plastic shroud which (a) keeps fingers away from the live busbar when the fuse is out and (b) prevents the casual (ab)user plugging 30 A fuses into 5 A circuits (notice how the width of the slots is graded). Undo the screw in the centre and it will just pull out - this exposes the live busbar, so turn off the power at the big switch first, unless you really know what you're doing. If you go to an electrical wholesaler and buy a new Wylex fuse assembly this shroud comes as part of it.
Having checked the terminal screws, offer the the new fuse carrier up to the fuseholder without the shroud in place first (power off) and make sure that the contacts are tight. The previous overheating may have annealed the contact springs to the point where the fuse is a loose fit. If this is the case you may have no alternative but to replace the whole consumer unit (in which case consider upgrading to a modern split load MCB unit). If you fit a new fuse assembly into loose contacts it will just burn again...
How sure are you that the "identical" replacement had the same power rating as the previous one? I strongly suspect your replacement is more powerful, as a fused one is likely to be at 30A, giving a maximum of 7.2kW. Of course, it is possible to get bigger fuses, but they aren't that common. Don't go fitting a bigger fuse unless the cable is found to be OK for the larger current.
Yes, but fuse boxes always seem to do this. After a few blows, they look like melted Mozzarella on a pizza. Consider replacing your consumer unit with a modern one, with RCD and MCB protection.
I'm concerned by most of the advice in this thread - not just the above. There coule be a live to earth fault and no bathroom bonding for all we know. Following any of the advice here could kill you.
You need to find out what the fault is, not start guessing. An electric shower is not something to muck about with.
I had the Aquatronic 8500 for a couple of years, it had blown the fuse once before and was cutting out all the time. I had the manual left from the previous owner and checked inside the housing for the rating. I wanted somthing I could replace myself, so bought the Aquatronic 2 from Wickes - for £60 or so with 2 years warranty. Everything was in the same place i.e water/cables etc so was a easy swap.
I was concious of the cable and looked along it length I can see (in the loft) any size mention, nothing written so measuring the size looked OK.
I went to my local electrical place an bought a 40amp Wylex Catridge (in the manual for the shower it says use a 45A an 6mm. I also got a 'cutting' of 6mm cable so I could compare. Ive taken off the shower cover and unscrewed the cable and measured it against, what I got given. They both look the same size, with multiple strands for the core and measure around 5mm with my ruler.
So I pretty confident the cable is good and the correct size.
I seem to remember that the fuse did seem quite loose and maybe that didn't help.
Taken from On Site Guide, Table 6D1 Method 3 (enclosed in conduit on a wall in trunking etc.) 6mm 2 cables single phase, current capacity = 41 amps
Method 4 (enclosed in thermally insulating wall etc) 34 Amps
Method 1 (clipped) 47 Amps
Most shower circuit's I've seen go straight up the wall across the loft and down to the shower, so some where in between Method 3 and 4, and at most going to be using 37 amps for max 30 min at a time.
So a 40 amp fuse will protect the circuit from any occurring faults.
But is it safe to make that assumption? Given that we don't know that there isn't a 5m run of cable running through loft insulation, is it safe to assume? I'm not being argumentative - I'm geniuinely curious about the correct way of deciding this.
Wrong hymn sheet, 6D1 (4D1A in the BS) is for singles, not twin & earth. For T&E look at Table 4D5A (6F in the OSG). For flat cable in insulated walls you choose from Method 6 (enclosed in conduit in insulated wall) - 32 amps - or Method 15 (installed directly in an insulated wall) - 35 amps. Method 1 (clipped direct) is the same rating though - 47 A.
Go into the loft and have a look. If the cable's surrounded in insulation the effective rating will be 20-something amps and trouble looms.
Ok, good point. Still finding my way around the on-site guide. Whoever put that document together could really do with some instruction on data structuring.
Yes it's abysmal, and poorly indexed too. And _still_ contains a reference to a shower cubical (/sic./) [3.6.1(v)] which appeared in the previous (blue) edition. It follows the formal structure of BS 7671 much too closely and hasn't been structured with the users' needs in mind at all. And the division of material between the body and the appendices is bizarre in the extreme.
I wasn't really sure what you were getting at there and just joined in too ;-)
I was really just responding to Dave Jones, who said: "8.5 kw shower will use approx 35 amps, check the cable if 6mm or above would be ok to just upgrade the fuse to 40 amp."
I was asking whether it would really be ok to fuse the circuit at 40A without knowing how the 6mm cable was routed. I personally feel uncomfortable using 6mm cable with anything more than a 32A breaker in any case, because you just don't know what might happen in the future (e.g. someone slaps a load on insulation on top).
An On-Site Guide is definitely needed, but the UK would probably be a safer place without that particular document. I'm a technical writer, so don't get me started...
Oh dear. Too late. You'd all better stay off the M4 for the next few hours.
It may not have been ideal (see other replies) but I have to say that putting 8.5kW on a 30A fuse is very common in my experience. The fact is that 8.5kW (approx 35.5A @ 240V) isn't going to blow a 30A rewireable fuse anytime this century - check out fig 3.2A in appendix 3 of BS7671 if you don't believe me. A 30A rewireable will only begin to think about blowing for a steady-state current of 55A or more (5 second disconnection is 87A).
Bear in mind too, all those who have been giving cable ratings for 6mm2 cable, that with rewireable fuses you have to derate everything to 0.725 times nominal (433-02-03) thus 6mm2 cable at method 1 which would normally give 47A is only rated to 34A.
I'd be doubtful that a wrong rating of fuse caused the burning originally described. As others have pointed out, this is far more likely to be down to a loose connection somewhere. Putting a 40A rewireable fuse in won't cure this and is likely to be (as I pointed out above) well outside the rating of the cable, even at the most lenient installation method. Ignore what I've said if you've actually installed a cartridge fuse (looks like a big plug fuse and says BS1361 on it) rather than a rewireable.
I would echo Christian's advice - at the least consider adding a separate CU containing an MCB for the shower even if you don't go for a full new CU. Screwfix do a couple of nice little units with RCD (may not be strictly necessary, but good backup) and MCB already installed -
69659 for example, though as this comes with a 50A MCB you'll either need to change to 10mm2 cable or swap the MCB for a lower-rated one.
Oh yes, and while you're at it, do make sure your supplementary bonding is up to scratch.
what we can deduce is that that is one of the possible scenarios. We can not deduce which scenario is definitely the one, the shower install will need to be tested to determine and correct the fault.
If the above scenario is what the OP has, which is possible, fitting a
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