Elecric shower blown fuse

[Andy Pandy]

My electric shower blew the cartridge fuse in the consumer unit while I was in it the other day, giving me a very refreshing cold shower. I replaced the fuse, and ran it for 10 mins or so, and all seemed OK, it didn't blow the new fuse.

Is this something I should be worried about, or do cartridge fuses sometimes blow with age? The old fuse (and shower) are probably about 12 years old.

It's a 30A fuse, I'm not sure of the power rating of the shower (it's a Heatstore Esprite II), but I guess it must be taking pretty close to 30A. But it has been working fine for at least 6 years.

I'm probably being over cautious, but the whole concept of a high power electrical device which has blown a fuse in close contact with water which gets sprayed on me has started to worry me. Would an RCD be advisable?

TIA

-- Andy

[Rumble]

Andy Pandy said the following on 11/02/2006 17:46:

Each time you use the shower, you take the fuse from zero current to maximum capacity (or more). This will cause the fuse wire to heat up and expand significantly, so I expect that the fuse died of thermal stress (i.e. old age).

You have not provided enough info. to calculate the disconnection time of the fuse in the event of a problem with the shower itself so I can't comment on whether you really should have an RCD regardless. Often with the larger showers installed these days, you *need* an RCD to meet the wiring regs requirements.

None the less, it is IMO always a good idea to protect your shower with an RCD, since it will likely protect you from a fault in the shower before the fuse gets close to popping. If you don't want the hassle of upgrading your consumer unit, you should consider a separate shower consumer unit with RCD protection.

HTH

[Harry Bloomfield]

Andy Pandy laid this down on his screen :

If the fuse blows again soon, then I would become concerned, otherwise it is likely that the fuse died of old age rather than an actual fault.

Irrespective of the above, an RCD would be a good idea anyway.

[Tony Williams]

We looked at a house the other day, which had had 3 en-suite showers fitted. I opened the cupboard to look at the fusebox (actual fuses), and stood staring at it.... firstly there were not enough fuses to support the number of en-suites, (one of them was shared!), and then I noticed not a single RCD in sight. :(

[Andrew Gabriel]

My hunch would be that the original shower was no more than 7kW, and has been replaced by one which is more, and is exceeding the fuse (and possibly wiring) rating. This will blow the fuse, but only after some time. Check the shower's rating plate. My second guess would be the element casing has failed.

Depends on the house wiring, type of supply, etc. It can never do any harm, but it might be completely unnecessary. Showers do not have to have an RCD unless the earth fault loop impedance is too high to guarantee the fuse will blow within 5 seconds of a short circuit to earth. If you don't have the tools to measure the earth fault loop impedance, you should assume it's too high to be on the safe side and fit an RCD.

What is important is to make sure all the supplementary bonding in the bathroom is correctly fitted.

[Andy Wade]

From the design point of view you can use Table 7.1 in the On-Site Guide. If the circuit cable length doesn't exceed the maximum figure given in the table, for the type and rating of fuse/MCB, the cable size in use and the type of earthing, you can be reasonably confident that an RCD won't be required. (TT installations excepted, of course.)

After installation you (somebody) must test the circuit and verify that Zs is OK. (OSG Appendix 2 Table 2A, B, C or D as appropriate.)

Absolutely. That's a much better step to take than unnecessarily fitting an RCD, IMHO.

[David Hansen]

On Sat, 11 Feb 2006 17:46:02 -0000 someone who may be "Andy Pandy" wrote this:-

Putting heatstore and shower into a search engine directed me very rapidly to

which reveals that the lowest rating of the current range is 7.2kW, 31.3A. Assuming that is the rating of your shower then the overload probably explains the eventual operation of the fuse. However, only you can tell us what the rating of the shower is.

I suspect it is a replacement for a lower powered shower and would wonder what the rating of the cable is.

The most important thing with an instant electric heater is that it is earthed properly. The innards must be solidly connected to the water pipe and this must be solidly earthed, separate from the "earth wire" in the cable supplying the shower.

An RCD may provide you with extra peace of mind. It will provide extra safety, but possibly not as much as some think. Depending on where the shower is, the electricity supply to the house and the nature of the fault, a cartridge fuse may operate while an RCD is still making its mind up.

[Andy Pandy]

The shower was in the house when we bought it over 6 years ago.

It's 8.5Kw! How has that been working for 6 years with a 30A fuse?

Would that not cause it to blow the new fuse pretty quick? I ran it for 10 mins after replacing the fuse and had a shower this morning and it has been OK.

5 seconds sounds like a very long time - wouldn't 240V for 5 seconds likely kill?

I might get a new consumer unit with RCD. Currently I've got 2 consumer units, one look fairly new with cartridge fuses, the other looks ancient and has the pull out rewireable fuses. I've got another shower that runs off the old CU.

I don't think there is any! Would it have been the norm to do this pre 1994 (when I think this room was converted into a bathroom)? I doubt there is in the other bathroom either, and that's got a shower over a metal bath!

One question about bonding - I can see it would make the bathroom safe but doesn't it increase the risk outside the bathroom (by potentially making more things outside the bathroom, eg radiators, live)?

-- Andy

[Andy Pandy]

The cable is 6.0 T&E (at least that what is says on the clips). The shower is

8.5Kw so I don't understand how it works with a 30A fuse.

The cold water pipe is earthed, but I'm not sure it's earthed near the shower inlet.

Isn't an RCD supposed to cut the supply much quicker than a fuse?

-- Andy

[Gary Cavie]

Fuses don't blow just above their current rating, they'll take an overload for quite some time. Take a look at

- This graph is for BS3036 rewireable fuses. Pulling 40A through it (on the bottom scale), and follow the line up - it meets (if ever) way, way off the graph. Even a

15A fuse would take 10s according to these curves!

However, running fuses under these overload conditions will cause them to age thermally, as someone mentioned earlier. A friend had a fault a few weeks ago, when he changed a light fitting. He connected all the reds together, and all the blacks together. When he turned it back on. and operated the switch, there was a muffled bang, and the service fuse (60A) blew, leaving the 6A MCB still in place. When the fuse was changed, it was one from out of the Ark, and had been used for so many years that the wire inside had, in all probability, been reduced to a hair-thin strand.

If the fault is a hard P-N short, the RCD won't even bat an eyelid at it

- both sides are balanced.

[Andrew Gabriel]

Well, it's only 35.4A.

Again, it could simply cause a little extra current draw.

I depends what you did with it. The time here is to limit the amount the cables heat up, so they don't damage themselves or cause a fire. You should not be in any danger during the fault regardless how long it goes on for, providing your bonding is to spec.

I wonder if anyone has calculated your demand loading against your supply. You can't normally get more than one electric shower on a

100A domestic supply.

Yes it was normal before.

No, except that if you have plastic pipework, you shouldn't bond radiators, baths, etc, as the plastic pipework (of at least 0.5m length) is actually a good insulator, even with water in it.

[Andy Pandy]

It's a BS1361 cartridge fuse - these blow much quicker than the rewireable ones don't they?

Yes, but a hard P-N short with no earth leakage would not usually present an electrocution danger would it?

-- Andy

[Gary Cavie]

Looking at the graph for that one (3.14b.gif), yes it does, but a 30A pulling 40A is still off the graph!

That scenario was in answer to the bit about RCDs blowing faster than fuses!

[David Hansen]

On Sun, 12 Feb 2006 19:58:42 -0000 someone who may be "Andy Pandy" wrote this:-

How is it installed? Is it run through thermal insulation, clipped to a wall, enclosed in conduit? Is it grouped with other cables?

There should be bonding between metal pipes, metal baths and electrical equipment in the bathroom. It can be concealed and hard to find, but if it is not there the installation is dangerous.

If there are several earthed metal pipes running to the bathroom, say hot/cold water and heating flow/return, and these are bonded to the shower, then that is solid enough earthing. However, changes to the plumbing may change this.

A common misconception. Fuses don't have mechanical parts which have to move to open the circuit. Depending on the supply and the fault, a fuse may disconnect a circuit far faster than an RCD.

[David Hansen]

On Sun, 12 Feb 2006 21:06:11 -0000 someone who may be "Andy Pandy" wrote this:-

A bit. However their main advantage is that they are less variable in their operation.

[Andy Pandy]

That would explain it! Thanks.

Yes, sorry, what I meant to write was that an RCD is likely to cut the supply quicker than a fuse in the event of electrocution (except of course in a pure P-N electrocution, which I guess is very rare).

-- Andy

[Andy Pandy]

A bit of all really! As far as I can tell, it runs through an internal wall with the cold pipe, up into the loft (insulated), then down through another internal wall and ends up in the cellar at the CU, at that end there are a load of cables going into a conduit.

I can't find any. Rather than taking floorboards up etc - can the bonding be checked by measuring the resistance? Are there any good on-line guides on how to do this? A simple test confirms electrical connection between the main metal items.

Indeed.

-- Andy

[Andy Pandy]

I did assume 35.4A for 10 mins through a 30A fuse would blow it...but it seems I was wrong.

Yes - I need to check this. Is this something fairly easy to do? Are there any good online guides?

Good point - I don't think the two showers have ever been used together. Put the oven on and boil a kettle and I guess that's pretty close to 100A.

The pipework is mostly copper, with a bit of lead.

-- Andy

[Dave Stanton]

Thats a bit over the top and a very sweeping statement.Have you seen the bathroom in question ?.The potential is there for it to be dangerous under certain circumstances true, but there are thousands of bathrooms up and down the country which have no bonding. And I have been to some where the bonding is infact incorrect. I don't think it was very wise to put the fear of god into people that they are taking their lives in their hands just having a shower.

Dave

[Ed Sirett]

I may have missed something here. I thought the reg was that supplementary bonding could be omitted if there was less than 0.5m of copper pipe?