Nightstor storage heater: Burnt cable

[piggoy]

We recently moved into a house that has an electric storage heater central heating system (GEC Nightstor). The system is powered from two 63A MCBs in the consumer unit, one a dedicated off-peak circuit (Economy 7) and the other a regular circuit. Upon testing we discovered the heat storage was not charging off the dedicated off-peak circuit. After isolating the system at the consumer unit and removing the cover of the control box I can see one of the cables for the off-peak circuit has completely melted and burnt. The cable that is black and burnt (picture below) was supposedly red originally! It has completely overheated and burnt. Based on the green colour of the oxidation of the copper at the isolator switch screw terminal I am assuming this damage occurred some time ago and the system as been inoperable ever since.

I would like to get the system repaired. However first I would like to understand how this damage could have happened. The cable that is burnt looks to be 10mm2 and is therefore more than capable of carrying the 63A current limited by the MCB. Perhaps the screw terminal became loose, but would that result in overheating of the entire cable length. Help appreciated.

The inside of the control box:

 

[JohnW2]

.... The cable that is burnt looks to be 10mm2 and is therefore more than capable of carrying the 63A current limited by the MCB. Perhaps the screw terminal became loose, but would that result in overheating of the entire cable length.

Copper is a very good conductor of heat. If overheating occurs due to a 'loose connection' at the end, the resulting heat can easily travel a significant distance up the cable, resulting in thermal damage.

 

[Harry Bloomfield]

Perhaps the screw terminal became loose, but would that result in overheating of the entire cable length. Help appreciated.

The heat can travel quite a way. You will need to replace both the cable, and whatever terminals it connects to. From the photo, it looks as if the upper terminal, which the black goes to, has also over heated.

 

[FrodoOne]

one of the cables for the off-peak circuit has completely melted and burnt. The cable that is black and burnt (picture below) was supposedly red originally! It has completely overheated and burnt. Based on the green colour of the oxidation of the copper at the isolator switch screw terminal I am assuming this damage occurred some time ago and the system as been inoperable ever since.

While the insulated conductor has overheated and melted/burned the insulation, it appears that the copper is still largely intact
and
(apparently) no circuit breaker (or other protective device) "tripped".

Hence, it maybe that whatever that conductor was "feeding" (such as a Heating Element) developed a fault
which, eventually, caused it to go open-circuit
thus eliminating the overload.

Because of this, I suggest that you first determine the item that this conductor is "feeding"
and check its "condition".
If it is now open-circuit, that would indicate that my thought is likely to be correct.

While copper is a good conductor it is unlikely that a loose connection at one end would cause such even overheating along that entire length.

The lower connector on the Left-Hand ABB B6-40-00 Contactor also shows signs of overheating.

​

 

[piggoy]

While the insulated conductor has overheated and melted/burned the insulation, it appears that the copper is still largely intact
and
(apparently) no circuit breaker (or other protective device) "tripped".

Hence, it maybe that whatever that conductor was "feeding" (such as a Heating Element) developed a fault
which, eventually, caused it to go open-circuit
thus eliminating the overload.

Because of this, I suggest that you first determine the item that this conductor is "feeding"
and check its "condition".
If it is now open-circuit, that would indicate that my thought is likely to be correct.

While copper is a good conductor it is unlikely that a loose connection at one end would cause such even overheating along that entire length.

The lower connector on the Left-Hand ABB B6-40-00 Contactor also shows signs of overheating.

​

Thanks. I guess two questions in my mind:

Firstly, if this damage is due to an overcurrent fault, why would a breaker not be tripped? Both of these circuits are fed from 63A breakers and constructed from 10mm2 cable throughout, the breaker should trip before any fault current is able to cause this level of damage through overcurrent heating?

Secondly, the circuit from the ABB contactors continues in the brown insulated 10mm2 cables. These show no signs of overheating from a supposed fault current.

I am leaning to the hypothesis this is damage caused by a poor terminal connection. In the paperwork for the new house I found an invoice for 2x ABB contactors that were replaced in 2019. My guess is the previous owner attempted a DIY repair on this and didn't torque up the screw terminal connections properly.

 

[Harry Bloomfield]

I am leaning to the hypothesis this is damage caused by a poor terminal connection. In the paperwork for the new house I found an invoice for 2x ABB contactors that were replaced in 2019. My guess is the previous owner attempted a DIY repair on this and didn't torque up the screw terminal connections properly.

That is the assumption I would begin with, if the damage is limited to that one cable, and it's terminals.

Heating elements, as the age, tend towards drawing less current as the age - due to surface oxidation, and increased resistance. Eventual element failure, occurs at locations where the element has got hotter, oxidised most.

 

[ericmark]

I wonder if the link wire is 6mm² even if the cable out is 10mm²? There is signs of burning both in and out of contactor, and a quick google seems to show 6mm² as rated at 41 amp, so an extended time load of 63 amp would likely cause damage.

The thought which comes to my mind, is what system uses 63 amp? My whole house is supplied with a 60 amp fuse. So I did some google work, it does seem to be a good system and I found an old post here and the post goes back to 2005.

It seems these were fitted around the 70's and 80's, so looking at a unit 40 to 50 year old. Looking at the diagram

I would say a good chance an element has failed. As to getting parts today, it seems unlikely, but found this dated 2024 so you may be lucky.

 

[FrodoOne]

The site https://www.pistonheads.com/gassing/topic.asp?h=0&f=207&t=1474867
indicates the following, concerning Nightstor '100':-

Input Element Rating- 19.2kW (Off-Peak) + 9.6kW (Boost)
Output at Full Charge- 22kW
Approx Weight- 750kg
Width(mm)- 610
Depth(mm)- 596
Height(mm)- 1664

19.2 kW (Off-Peak) would be a current of 80 A at 240 V - which I cannot believe !
even the 9.6kW (Boost) would be a current of 40 A
and
the ABB B6-40-00 Contactor, 24 V Coil,
is a 4-Pole, 20 A contactor
with the 4 Poles wired in parallel

However, there are/were also Compact 45 or Compact 65 Nightstors, which (presumably) had lower capacities.
It would be of interest to determine which model "Nightstor" is involved in this case.

 

[piggoy]

The site https://www.pistonheads.com/gassing/topic.asp?h=0&f=207&t=1474867
indicates the following, concerning Nightstor '100':-

Input Element Rating- 19.2kW (Off-Peak) + 9.6kW (Boost)
Output at Full Charge- 22kW
Approx Weight- 750kg
Width(mm)- 610
Depth(mm)- 596
Height(mm)- 1664

19.2 kW (Off-Peak) would be a current of 80 A at 240 V - which I cannot believe !
even the 9.6kW (Boost) would be a current of 40 A
and
the ABB B6-40-00 Contactor, 24 V Coil,
is a 4-Pole, 20 A contactor
with the 4 Poles wired in parallel

However, there are/were also Compact 45 or Compact 65 Nightstors, which (presumably) had lower capacities.
It would be of interest to determine which model "Nightstor" is involved in this case.

You are correct! The unit in my house is the Compact 65. I simply said Nightstor because it is a brand of storage heater from GEC that people are more familiar with. Furthermore, my unit isn't even made by GEC, it was made by a company called APECS who were a spin off from GEC when the former decided to cease production. My unit was fitted brand new in 1998 when the house was built.