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Discuss Immersion tank, element fault, hot lives, heating 13A fuse in the DIY Electrical Forum area at ElectriciansForums.net

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Delectrician

EF Member
Hi,

I have a fault somewhere on my immersion heater circuit, the 13A fused spur switch is getting hot & there is visible insulation damage to the live conductors only, the fuse in the spur is where the heats being generated, I disconnected everything & tested the resistance of the element alone, its a 2.8KW rod & I recorded a value if 18.5 Ohms, Fairly normal for an element that size, I thought!

The circuit is governed by a 16A MCB, through a 80A RCD unit, on a duel RCD fully loaded board, I tested the functionality of the RCDs & both are operating as normal, I am suspicious the fault is the element, but with no faults to Neutral or Earth, I'm a little perplexed! The property was EICR tested only 1 year ago by an NIC EIC registered company, so all the continuity values, IR readings & loop impedance have been recently checked.

Any help would be brilliant, I don't want to pay for the element to be replaced & find out It wasn't a fault with it!

Thanks in advance!
 

Matthewd29

Regular EF Member
What size of cable is it? Maybe it would be better with a 20a Double pole water heater switch rather than a 13a fuse spur as at 2.8kw your not far off 13a so it will heat up if on for long periods of time
 

Delectrician

EF Member
What size of cable is it? Maybe it would be better with a 20a Double pole water heater switch rather than a 13a fuse spur as at 2.8kw your not far off 13a so it will heat up if on for long periods of time

Its 2.5mm cable, if the element was pulling above 13A for a long time then Id expect the fuse to blow, but It is instead just creating loads of heat & not blowing, the last switch had melted the plastic fuse holder to the fuse but never popped, I replaced the whole switch today and after 10 minutes on the new fuse was very hot, so I isolated & left to come her for advice, by my calculation there should still be an Amp headroom, with the 13A Slow blow, its a very short run of cable.
 

Delectrician

EF Member
The fault is the Fused Spur, bad contact between fuse and carrier.
The fuse holder won't take 13amp continuously for the length of time the heater is on.
20amp DP. is the answer as above.
This is the 3rd one in a row with the same fault then, as EICR company changed it out for the same reasoning (ie it was a faulty switch), then it began to heat again, so I changed it out after testing the elements & RCD function & began to pull heat to that fuse again, Can it be possible that 3 switches in a row have the same problem, or is it more likely that the element is drawing above its rated amount, even if so, why is the fuse not just blowing? Why is it getting so hot that its melting plastic but not breaking continuity?
 

SparkyChick

Making a banana smoothy for my fave gorilla
Staff member
Moderator
Fuses blow by getting hot. Run a fuse close to it's rating (which you are doing) for extended periods of time and it will get hot, but sit there quite happily.

And, running it close to it's rating assumes a voltage of 240v. More than that and you'll be running on the raggedy edge.

You've already received the best advice... lose the switched fused connection unit and replace it with a 20A double pole switch.
 

buzzlightyear

please let me back in to the prison cell.
Electrician's Arms
What size of cable is it? Maybe it would be better with a 20a Double pole water heater switch rather than a 13a fuse spur as at 2.8kw your not far off 13a so it will heat up if on for long periods of time
you bang on the monie there. drawing to much current on that e/heater
 

Delectrician

EF Member
Fuses blow by getting hot. Run a fuse close to it's rating (which you are doing) for extended periods of time and it will get hot, but sit there quite happily.

And, running it close to it's rating assumes a voltage of 240v. More than that and you'll be running on the raggedy edge.

You've already received the best advice... lose the switched fused connection unit and replace it with a 20A double pole switch.

Thanks

This makes sense!
 

Murdoch

Electrician's Arms
problem these days are the carp switches and when the voltage drops, the amps increase .... so a 3KW [email protected] 250 v pulls 12a
@ 240v pulls 12.5a
@230v pulls 13a
@220v pulls 13.6a
@212v pulls 14.5a (lowest voltage I've seen recently)

then allow for poor connections in the switch / fuse
 

123

Electrician's Arms
problem these days are the carp switches and when the voltage drops, the amps increase .... so a 3KW [email protected] 250 v pulls 12a
@ 240v pulls 12.5a
@230v pulls 13a
@220v pulls 13.6a
@212v pulls 14.5a (lowest voltage I've seen recently)

then allow for poor connections in the switch / fuse
You sure about that? The resistance of the immersion element coil is constant, so as the voltage drops, so should the current.
 

Marvo

Admin and gender confused
Staff member
Admin
problem these days are the carp switches and when the voltage drops, the amps increase .... so a 3KW [email protected] 250 v pulls 12a
@ 240v pulls 12.5a
@230v pulls 13a
@220v pulls 13.6a
@212v pulls 14.5a (lowest voltage I've seen recently)

then allow for poor connections in the switch / fuse
I'm going to disagree, if it's a linear load of fixed resistance the current decreases as the voltage decreases. A 3kW element is only 3kW at it's rated voltage. At a lower voltage it has a lower kilowatt output. I would however agree with the statement about poor manufacturing quality.
 

telectrix

Scouser and Proud of It
Respected Member
agree with marvo and 123. lowering the voltage will reduce the current and hence also reduce the wattage.

i = V/R, so for a 18 ohm element...

250V,.... I = 250/18 = 13.888888888A

230V .... I = 230/18 = 12. 77777777777A
 
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