# Why does my shower switch keep burning out?

I have a Mira Shower 10.8 kW.

My switch is a 45A switch. I have a 50A fuse for the shower. The shower uses 10mm2 cable.

9 months ago the switch sparked and refused to turn off and the fuse tripped. Opening up the switch, the cables were black and burnt. I replaced the switch. 9 months on and it's happened again.

I know you're probably supposed to cut the cable and expose copper but I have no length left so I'll have to clean it with a wire brush and replace the switch.

Why does this keep happening though?

• Wow, I didn't know electric showerheads were a thing. I don't think I'd be comfortable with that, especially with such high overcurrent protection... Commented Sep 9, 2013 at 20:40
• @BlueRaja-DannyPflughoeft in the UK (unlike some third world countries) the heater/control part of an electric shower is wall mounted and the shower head itself is just a simple showerhead (usually on a flexible hose). Commented Jan 27, 2016 at 16:37

Mira seems to be based in the UK, so I'm assuming a 230V electrical mains.

Just taking the nominal figures for power and voltage gives: 10.8kW / 230V = 46.96A. If you look at the worst case mains voltage (230V - 10% = 207V), you could actually be drawing over 52A.

So, if your shower is drawing as much power as it's rated for (say if the incoming water is very cold, or it's a cold day and you've got the water temperature turned up), a 45A switch is actually underrated for the job it's doing. I'd recommend replacing it with a heftier switch.

• We're 240v in the UK. How does the math look now. It might still suggest the same. Thanks
– Jon
Commented Feb 20, 2011 at 18:18
• Crap just check Wikipedia and we're 230. Could have sworn we were 240
– Jon
Commented Feb 20, 2011 at 18:56
• Mains in most of Europe is 230 within 10%, which means technically 207 to 253V is allowable. Some EU countries were previously 240 (now 230 +10/-6%) and some were 220 (now 230 +6/10%), which lets them continue to generate electricity at the same voltage as before, but still meet both their historic and the new EU guidelines. (en.wikipedia.org/wiki/Mains_electricity#Standardization) Commented Feb 20, 2011 at 21:31
• "If you look at the worst case mains voltage (230V - 10% = 207V), you could actually be drawing over 52A." heaters are largely resistive so to do sensible current calculations you need to know what voltage the rated power is quoted at (usually 240V for UK showers in my experiance) and work back from there to the resistance. The worst case with a resistive load is overvoltage not undervoltage. Commented Jan 27, 2016 at 16:03

Because it sparked, my guess is the switch you're using is not big enough. The pull-cord style double-pole switch at 45A is foreign to me (I have never seen anything like it in north america), but the specs on the Mira site say to use a switch with at least 3mm of contact separation.

This separation is important, because at these currents there is going to be arcing every time the power is toggled, which can do several things that negatively impact any type of switch/relay:

• "Burn" the terminal pads, which scorches them with the same black you see on the burnt wires outside, and over time, increases the resistance and thus increases the current draw through the pads, leading to even worse arcing
• Weld the terminals together

My guess with the info provided is both of the above happened in this case. The burn on the wires is probably caused by the arching inside the switch: maybe there is inadequate mechanical separation between the contact pads of the switch and the outside terminals?

My inclination, if I were to ever install something like this here, is that 45A is too much for any type of user-facing switch that will be used on a regular basis. I would install a contactor (basically, a big relay) separate from the switch, which is rated at at least 60A (since as Nial C pointed out, depending on the exact voltage, you could be seeing as much as 52A). Then I'd run a separate circuit to the pull-switch (or whatever you're using) that simply controls the power to the contactor coil. This circuit would only have a minimal current in it (<1A) required to run the contactor coil.

Here's a quick diagram hacked together based on the Mira install manual:

Note though, I do not know if this would comply with EU plumbing/electrical codes! It seems a whole lot safer to me as it keeps a human from having to interact directly with a high current switch, and it's likely easier to replace the contactor if needed (though I'd guess it's less likely to be needed, as contactors are very commonly used in industrial systems to control high-current loads for many years without issue).

• I never did understand why people like putting high power circuits right near the people ... but as Star Trek has shown us, it'll still be happening centuries from now, even though someone could've use a low voltage switch at the people, that triggered a relay some safe distance away.
– Joe
Commented Feb 21, 2011 at 2:58
• Thanks. This is the switch that is used at the moment. Maybe a pull cord would be better - bit.ly/fZQ7bR
– Jon
Commented Feb 21, 2011 at 9:12

Seems strange to me to have a 45A switch and a 50A fuse (is that an MCB in the consumer unit?). What is protecting the switch from overcurrent? If you have a component that is rated to 45A I think it should be protected by at most a 45A fuse/MCB. If your 45A switch is only protected by a 50A fuse, then you could be drawing a dangerous overcurrent through the switch for too long before the fuse/MCB will detect a problem. So just on that basis I think you might need to either put in a smaller fuse/MCB or a higher rated switch (actually a higher rated switch, as your shower can draw more than 45A).

Another thing that is worth considering is the rating of the 10mm2 cable. Although 10mm2 cable can be used for (e.g.) 50A currents it is only rated that high if there are no derating elements to consider. For example if it is run inside insulation or next to other cables then its rating is lower. 10mm2 run by itself inside conduit in an insulated wall is only rated to 46A.

Personally I would seriously consider putting in a less powerful shower. I did that in my current house when I found that the cable and switches were not rated high enough. (the shower was pretty old and dodgy anyway so replacing it was not a big issue).

Another thing that might affect you is the size of the main fuse to your house. Some older houses only have a 60A supply.

• I think a higher rated switch is going to have to be the way forward. I can only find a 50A switch online, can't seem to find a 60A for example
– Jon
Commented Feb 21, 2011 at 15:18

If your cables are burnt it is not a faulty switch, it is poorly installed. The reason your cables are burnt is because heat has built up in the connection to the switch. The cable must be clean and securely clamped to ensure minimum resistance.

The formula for heat dissipation is I^2 * R. In your case, I = 45A. With just 0.1 ohms of resistance at the screw terminal you will create over 200 Watts of heat and the poor old terminal will not be able to get dissipate it. The end result is that it will burn the cable.

The fix is to clean the conductors and screw them down tightly.

• This. For 45 A cables they have to be connected really well. I would be very surprised if installing it yourself is even legal.. (and for good reasons from a fire safety perspective).
– jpa
Commented Nov 18, 2014 at 6:32

I had the same problem with a 10.8kW Mira. Initially I used a 45A d/p pull switch, protected by a 63A Residual Current Device (RCD) and a 50A Miniature Circuit Breaker (MCB). The switch burned out.

I then purchased a 50 A d/p pull switch. Nine months later the switch burned out again and the 63A RCD had burned out. I used a 10mm twin and earth cable. Also the cable run was quite substantial, approximately 30 metres, so voltage drop could be an issue.

Your RMS or root mean square voltage in the UK does fluctuate between 220V and 250V so check that out and do a calculation to see how many amps you are actually drawing. (power over volts)

(this post assumes you are in the UK, elsewhere YMMV)

This is usually caused by poorly terminated wires which leads to the terminals and wire being overheated.

The problem is overheating doesn't just tarnish the surface, it can also change the physical properties of the wire. Cleaning the tarnishing off is not enough the wire MUST be cut back. The terminations must be on fresh wire that has not been overheated and they must be tight. Failure to do this will almost certainly result in a repeat of the problem.

If there is not enough slack in the wiring then that needs to be dealt with by replacing wiring. Something like this 60A junction box with double screw terminations would be appropriate would be appropriate if you need to join cables but if the cable can be easilly replaced completely then that would be preferable. Remember junction boxes must remain accessible for inspection.

I would advise moving up to a 50A switch if possible to give a bit more headroom.

Personally I think the whole design of accessories for cooker and shower circuits in the UK is one of the less well designed parts of our wiring system. Fitting the accessories onto their backboxes requires moving the wiring which with fat inflexible cores often means some stress (especially if the wall is solid and the cables are direct burried so they can't move).

There is no law in the UK which would prohibit DIYing this work but if you are at all unsure of your abilities I would advise using the services of an electrician.

P.S. a note on power, voltage and current because a lot of people get this wrong.

Many people have heard the equation I=P/V and try to apply it in circumstances where it doesn't apply. Specifically the headline power figure for a resistive load (like a shower) will only be valid at one voltage. For UK electric showers the headline power is usually specified at 240V (despite the fact that our mains electricity is nominally 230V).

So to caclulate the current at a given voltage you would first calculate the resistance from the headline power and the voltage at which the headline power is specified (using R=V2/P). Then you can calculate the current from the resistance (which we can assume is constant over a reasonable range of voltages) and the voltage you want to know the corresponding current for.

I have the same problem. For over 3 years every 3 to 4 months I have to change the electric switch. Sometimes twice in one week.

I've spent more then electric shower on buying switches and labour cost for electrician on the top of call out charge for Triton.

Here are my observation hope it helps someone who are in same boat as me.

• It happens mostly when used two or more people subsequently
• The low pressure of water mainly when its connected to main water instead of tank where when some one turns on kitchen water during shower
• Something is wrong with solenoid valve once when i replaced it worked almost for 7-8 months

I am going to buy the new one this week.