.......fake fuses query......

Hi all.
Just to update you, the thermostat on the boiler works. I turned the temperature dial up, the red light came on to heat the water, 5 minutes or so later the red light went out.
I have attached a couple of photos of the fuse board. The MCB marked Hot Water is the boiler, which looks like a 16amp fuse to me?????
I have also noticed that the 13amp FCU is fitted inside the boiler cupboard and recessed in to the external wall using a back box. The front door is next to the boiler cupboard and if you shut the front door with a little bit of force then the wall shudders slightly. Just thinking along the lines of what was previously mentioned about slightly poor fitting fuses inside the FCU and being exacerbated by being sited 20cm from a slamming front door. Picture attached should show/explain this more clearly.

P.S. Not sure if the fuse is fake.......apologies for misleading anyone, but you all sound like you are enjoying the research opportunity. Your knowledge and help is appreciated.
 

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ban-all-sheds said:
That "I believe the standard 13 amp plug is not rated to operate normally at 70°C" is of no relevance, because we know that when used in an FCU the conductors, and therefore the fuse holder, and therefore the fuse end caps, and therefore (at least some of) the fuse wire may legitimately reach 70°C.
Click to expand...

Do you really believe that a reputable manufacturer would produce fuses that could get hot enough to damage a plastic moulded plug and excuse that by saying that the fuse was permitted in a fused device ( such as an FCU ) that could "legally" operate at 70°C.
 
Tudor said:
I have attached a couple of photos of the fuse board. The MCB marked Hot Water is the boiler, which looks like a 16amp fuse to me?????
Click to expand...
Yes 16A MCB (not fuse).
You could replace the FCU with a 20A switch.

I have also noticed that the 13amp FCU is fitted inside the boiler cupboard and recessed in to the external wall using a back box. The front door is next to the boiler cupboard and if you shut the front door with a little bit of force then the wall shudders slightly. Just thinking along the lines of what was previously mentioned about slightly poor fitting fuses inside the FCU and being exacerbated by being sited 20cm from a slamming front door. Picture attached should show/explain this more clearly.
Click to expand...
Could be. It won't help.
Just make sure the terminals are done well and tight - and the fuse holder.

P.S. Not sure if the fuse is fake
Click to expand...
It can't be any worse than some of the things being discussed.

.......apologies for misleading anyone, but you all sound like you are enjoying the research opportunity. Your knowledge and help is appreciated.
Click to expand...
It's always like that.
 
bernardgreen said:
Do you really believe that a reputable manufacturer would produce fuses that could get hot enough to damage a plastic moulded plug and excuse that by saying that the fuse was permitted in a fused device ( such as an FCU ) that could "legally" operate at 70°C.
Click to expand...
It's not a question of how hot the fuse gets because of its own properties.

The point is that you have conductors, terminals, fuse holder, fuse end caps and fuse wire, all metal, all in good contact with each other, and all good conductors of heat.

If the conductors reach 70°C then all the other components are likely to.
 
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This will come as a surprise to you.

Conductors will reach 70°C when carrying the tabulated current for their installation method.

That is not an overload.
 
Last edited:
ban-all-sheds said:
Conductors will reach 70°C when carrying the tabulated current for their installation method.
Click to expand...

Tabulated current is the maximum safe current for the cable.......it is not the designed current for the cable.

. For cables that are not buried, the tables assume an ambient temperature of 30°C and a ground temperature of 20°C for buried cables. For PVC insulated conductors, the maximum current carrying rating assumes a conductor temperature of 70°C.

These ratings assume that the maximum current for the cable is continuous for an extended period. 70°C is very hot and at this temperature the PVC insulation will not be damaged but will degrade over a shorter period of time than a cable run at a lower temperature.
Click to expand...

How often will a cable be carrying the maximum current in an ambient temeprature of 30°C.
 
bernardgreen said:
Tabulated current is the maximum safe current for the cable.......it is not the designed current for the cable.
Click to expand...
One might have thought so, and it certainly would make sense for there to be deliberately-included 'safety margin' between what was regarded as 'the maximum safe current' and the current which would be expected to be result in the cable's rated conductor temp (e..g. 70°C).

However, as recently discussed here, it is clear that some of the calculations presented in BS7671 (and in associated IET-related publications - such as the equation for VD calculation presented in Appendix 4 of BS7671) assume that a conductor will reach the cable's rated temp (e.g. 70°C) when the 'tabulated maximum current' is flowing. (that, amongst other things, makes me feel very sceptical about that equation).
How often will a cable be carrying the maximum current in an ambient temeprature of 30°C.
Click to expand...
That's another seemingly silly thing about that equation - although BS7671 is produced primarily for the UK, it says that the equation only applies for ambient temps of 30°C or higher!

Kind Regards, John
 
Last edited:
JohnW2 said:
it says that the equation only applies for ambient temps of 30°C or higher!
Click to expand...

The "equation" is constructed in such a way that the cables in an installation that is compliant with BS7671 will be "safe" on a fully worse case situation. Therefor it has to consider an ambient temeprature of 30°C as possible in the worse case situation.
 
bernardgreen said:
The "equation" is constructed in such a way that the cables in an installation that is compliant with BS7671 will be "safe" on a fully worse case situation. Therefor it has to consider an ambient temeprature of 30°C as possible in the worse case situation.
Click to expand...
The equation is not really anything much to do with 'safe', since it is only about calculating/estimating voltage drop.

In any event, the equation is surely considering anything but the 'worst case' (in terms of VD). It contains no term for ambient temp, merely stipulating that the equation only applies for ambient temps of 30° or above. The worst case (for VD) would surely be with an ambient temp of, or approaching, 70° - yet the equation would give the same answer for that as with an ambient of 30°?

Another iffy thing about the equation is that it assumes that conductor temp will be proportional to I². Whilst heat production will obviously be roughly proportional to I² ('roughly' since the whole point of the exercise is that resistance will change with changing current!), but what that means in terms of temperature change is a far more complicated question.

Kind Regards, John
 
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