Immersion Heater Timer

[Doubleo]

https://www.electricalsafetyfirst.org.uk/guidance/safety-around-the-home/plugs-and-fuses/ says
"Plugs for appliances rated between about 700 watts and 3000 watts (the maximum rating of a wall socket) should be fitted with a 13-amp fuse (coloured brown)."

There are various calculations online. Some take UK voltage at 230 and some take it at 240 . If 230 then they suggest multiplying by 1.25 ... (watt/volt) x1.25
If 240 then they leave it as is...


But i understand that the cable rating also needs to be taken into account when deciding on the value of a fuse...
Though these are for plugs (which i think in my case is the same as a switch)....

is that right ?

this is all fascinating btw...

 

[Doubleo]

240 volts divided by 17.7 ohm = 13.56 amps

So , either way, a 13 amp fuse is in the original switch. Which I'm guessing is fine. (?)
The new timer-switch has also a 13A rating (for a resistive load) and a 13A fuse.

 

[JohnD]

immersion heater circuits are usually fused at 16A (they should not be on socket circuits), and their timers don't usually have fuses in them.

examples
https://www.tlc-direct.co.uk/Main_Index/Timers_Index/Timers_Wired/index.html



But I expect it will be OK.

 

[Doubleo]

Thanks John... I understand that if a fuse is lower than necessary, it runs the risk of 'blowing' if there is a surge...
So in a way, the lowest possible fuse is 'safer' but runs the risk of cutting out....
Am i right?

In which case, the current switch I have has 13A fuse anyway. And it's a simple on/off switch. I have no other means to turn it on or off. It was like that when I moved in. (Which i'm guessing is not ideal)....

I've learned so much through this discussion... (Though it has distracted me from my own work!!)

 

[EFLImpudence]

But i understand that the cable rating also needs to be taken into account when deciding on the value of a fuse...
Though these are for plugs (which i think in my case is the same as a switch)....

is that right ?

The cable rating is all that needs to be taken into account.
Fuses and MCBs are to protect cables; not appliances.

13A is the maximum available for UK plug fuses.

 

[Taylortwocities]

Thanks John... I understand that if a fuse is lower than necessary, it runs the risk of 'blowing' if there is a surge...
So in a way, the lowest possible fuse is 'safer' but runs the risk of cutting out....
Am i right?

Don't worry about that, The actual current, where a BS 1362 fuse will blow, is about 1.66 times the rated value, so for a 13A fuse this will be 21.6A.

 

[Doubleo]

Thanks everyone ! I am biting the bullet

 

[terryplumb]

What brand/ make is it ? Is it GreenBrook (kingshield)

You didn't respond to this ,but nevertheless you have a fused ,switched connection unit ,and you can replace it with the timer which is effectively a fused ,switched connection unit with integral timer.And to answer your original question ,the earth conductors go together into the terminal on the back of it.

 

[winston1]

With a resistive load, the amps go UP when the volts go up.

Working from the nominal figures:

We know that the nominal voltage is 230

We think that the nominal amps are 13

x volts ÷ y amps = z ohm

230 volts divided by 13 Amps = 17.7 ohm (rounded)

Calculating on 240v:

x volts ÷ z ohm – y amps

240 volts divided by 17.7 ohm = 13.56 amps

Here we go again. The UK voltage is 240v and has been since nationalisation. (Some idiots decided to call it 230v a while back but nothing has changed). His immersion heater is most probably rated at 3kW at 240v so Harry Bloomfield is correct.

 

[JohnD]

Harry Bloomfield is correct.

Really?

 

[EFLImpudence]

Yes, Harry and Winston are correct.

With a resistive load, the amps go UP when the volts go up.

They do.
That means they go DOWN when the volts go down.

Working from the nominal figures:
We know that the nominal voltage is 230

We do.

We think that the nominal amps are 13

We do not.

x volts ÷ y amps = z ohm
230 volts divided by 13 Amps = 17.7 ohm (rounded)
Calculating on 240v:
x volts ÷ z ohm – y amps
240 volts divided by 17.7 ohm = 13.56 amps

What about?

x volts ÷ y amps = z ohm
240 volts divided by 12.5 Amps = 19.2 ohms (exactly) and the approximate resistance usually found in the elements.
Calculating on 230V:
x volts ÷ z ohm – y amps
230 volts divided by 19.2 ohms = 11.99 Amps.

 

[EFLImpudence]

2760W ÷ 230V = 12Ω

 

[Harry Bloomfield]

Thanks All and especially EFL for the photo evidence to confirm that what I knew was correct, was in fact correct. I have not seen many immersion heaters, but all of those I have come across have been rated 3Kw at 240v. I have come across JohnD's argument that the rating is 3Kw at 230v before, but I have never seen such an immersion heater in the UK.

 

[Harry Bloomfield]

immersion heater circuits are usually fused at 16A (they should not be on socket circuits), and their timers don't usually have fuses in them.

As is my immersion circuit a 16amp MCB, but that is more to do with the fact that they don't manufacture a 13amp MCB. The 16amp is to protect the 2.5 T&E and the final section of 2.5mm flex. It isn't to protect the 240v 3kw 12.6amp immersion heater element.