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

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Reaction score
1
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!
 
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.
 
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.
 
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?
 
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.
 
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
 
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!
 
problem these days are the carp switches and when the voltage drops, the amps increase .... so a 3KW heater@ 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
 
  • Disagree
Reactions: 123
problem these days are the carp switches and when the voltage drops, the amps increase .... so a 3KW heater@ 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.
 
problem these days are the carp switches and when the voltage drops, the amps increase .... so a 3KW heater@ 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.
 
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
 
problem these days are the carp switches .......
I agree with the first bit .
The Quality of current parts -is soon shown up by this circuit !
( possibly running in a warm enviroment )
If spare element is rated at 230V and being given 240V ..may be running a higher current than in past .
(Its not a switched mode PSU --YET)
 
Last edited:
This is the 3rd one in a row with the same fault then ... or is it more likely that the element is drawing above its rated amount ... why is the fuse not just blowing? Why is it getting so hot that its melting plastic but not breaking continuity?
A couple of thoughts :
- maybe all 3 are carp brands? But from what I've seen I'm inclined that none of them are any good for 13A continuous nowadays.
- a 13A fuse definitely won't melt at 13A but will be warming up for it :)

Immersion tank, element fault, hot lives, heating 13A fuse IMG_0416 - EletriciansForums.net
 
[QUOTE
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.]


A few observations and remarks:

1. The EICR was a year ago. Had the IR of the element, L-E, N-E been measured before the last FCU was replaced?

2. The IR measurement must be done when the thermostat contacts are closed - A year ago it may have been that the contacts were open during the measurement. Only sure fire test is to test IR by connecting to the element's terminals.

3. It's time to AMPCLAMP one of the conductors supplying the IH to establish how much current is actually flowing. Measure the voltage too across the element.

4. Is the thermostat actually opening when the required HW temperature is reached?

5. Is the element cycling on and off - this can happen if the element/thermostat is at the bottom of the tank and it is covered in limescale and detritus or if you have a hot water leak somewhere. The resistance of the element will increase slightly as it heats up to its operating temperature but during this warming up period a higher current than the running current will flow causing more Ohmic heating of the fuse.

6. The thermostat needs to be a modern one which includes a safety cut out as well as the normal bimetallic contacts. Is the thermostat the correct length for the element and the element the correct length for the tank?

7. Is your water being heated to a temperature above 60C? Is it steaming/scalding hot when you draw hot water. If so, the element is on longer than usual and heat loss from the tank will be higher too requiring more heating from the element.

8. Is the tank lagged and lagged with the latest low heat loss thermal insulation?

The rated power dissipation of a BS1363 fuse is 1W at 13A. Modern plastic FCUs unlike the older ones tend to have higher thermal resistance to heat flow between the fuse and the surrounding air which means they can often run hotter, especially if the ambient is high too as is the case near an IH. And they are white - black is better.
 
Last edited:
@marconi . 4. Is the thermostat actually opening when the required HW temperature is reached?

I had this just recently on an old immersion heater . The thermostat was burnt to a crisp inside , although it was still functioning to a degree it was enough to melt the insulation from the the stat to the element.
The water was heating but not to the selected temperature.
 

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

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock