Fusing the neutral?

[bilge rat]

Baxi have now replied. They say (as Snowhead says) that as this is an appliance, it does not come under the juristriction of the IEE regs. They have however agreed to errors in their circuit diagrams which I had pointed out and state they will amend
their diagrams. I still think fusing the neutral is a bad idea. In reply to darkwood, my main concern is that if a neutral fuse or
circuit breaker disconnects, the appliance may appear dead whilst still having 230 volts across it. Also as no load is being carried by the appliance, there will be no volt-drop. This results in all neutral connections within the appliance becoming live between neutral and Earth.:icon13:

 

[Marvo]

They probably sell the same model in the US and it's connected across 2 phases.

I agree internal fusing of the neutral shouldn't be hazardous, if you remove the covers of any electrical appliance it should be isolated externally beforehand so no chance of shock.

 

[Darkwood]

Yes Bilge i agree with what you are saying but the wiring and circuitry following the both N and L fuses are so designed that either fuse rupturing wont create a dangerous situation, the use of transformers etc or galvanic isolation will mean a hazzard isn't posed, with regards to Mr sparky coming in to check boiler he will surely if competent check the supply to boiler and subsequently the fuses too, the faulting of the circuit board would be by someone trained to fault or repair that particular board but in general service engineers just check relevent supplies at specified points then replace whole board instead of repairing it. I often build control panels and its a whole different kettle of fish not covered by 7671 and where Electricians connect to or fault a circuit board it is all too common to assume bs 7671 applies to the circuitry 'boiler in this case' itself.

 

[Darkwood]

If you are still confused consider this, a boiler control board is usually one of 3 types:-
-power to terminals 230v then straight to a transformers and all controls are selv and a fusing of the neutral would stop the transformer working and all control power lost.
-power to terminals 230v, terminals for time-clocks and/stats etc all running off supply terminals then to control board, again the board with be designed if it neutral fused so that a fusing on the N will not effect any outgoing circuits like timeclock and stat but will only effect power to circuit board thus stopping boiler operating only.
-lastly a mixture of the above but again no dangerous situe would arise if N is fused and it operated.

To note this expln' is only relating to boilers where you may find a N been fused, other designs exist where fitting a N-fuse would create a dangerous situe but obviously these dont have one to start with due to there design.

 

[bilge rat]

Thanks for that darkwood. Your explanation sounds plausible to me. Still not sure I understand the need or advatage of fusing the neutral at the input, it is after all, essentualy connected to Earth at the supply authorities end.

 

[Darkwood]

In your defense many yrs ago i had the same head scratching time understanding it but its second nature now to me, and in response its part of the nature of how transformers can fail and also how electronics work in that fusing the neutral can avoid certain situations that may overheat a board or even prevent a burn-out although its a bad way to describe it but the easier way to explain forget the neutral is strapped to earth or earth exists for a minute then you only have a p.d. between the line and neutral it could be thought of as 2 incoming phases (although its not the same) a circuit board fault can overload either N or L so both need fusing to avoid fire etc

 

[RobWansbeck]

To elaborate on what some others have said. Equipment that is
permanently connected to a supply or exclusively uses a polarized
plug requires one fuse in the live connection. Equipment that might
be connected using a non-polarized plug must have a fuse in live and
neutral.

From a repair viewpoint you should disconnect the supply before removing
the covers but still assume that everything inside the box is live
until you know better due to the possibility of charged capacitors.
You would then look for obvious burnt-out components and check the
fuses before you reconnected the supply so there should be no issue
with safety.

 

[Darkwood]

Equipment with fused neutrals is usually beyond the scope of your standard electrician and usually requires knowledge in other fields if not specific training to said boiler in this case, the spur or switch feeding the boiler above would have requested in the install intructions a DP isolation local to boiler which goes without saying why isolation off neutral maybe required before opening up the boiler for inspection, maintenance or repair.

 

[bilge rat]

Equipment with fused neutrals is usually beyond the scope of your standard electrician and usually requires knowledge in other fields if not specific training to said boiler in this case, the spur or switch feeding the boiler above would have requested in the install intructions a DP isolation local to boiler which goes without saying why isolation off neutral maybe required before opening up the boiler for inspection, maintenance or repair.

Yes Darkwood, I agree that often, the inner working of an appliance may not be understood by a standard electrician. However, this was not a standard electrician doing the wiring but a plumber. He had wired it incorrectly but, in his defence, the circuit diagram was incorrect. 'Baxi' have acknowledged this and say they will change the installation diagrams.--- No, the circuit connecting the 'baxi' to the supply does not ask for a double pole switched unit to be installed. in fact, they show it being connected to a single pole fused spur unit. The plumber did what plumbers tend to do, that is, connect the boiler to a socket outlet via a 13a plug. As the socket also feeds a washing machine, the washer has to be lifted out from base units before you can get at the 13a plug. Not easy due to its weight and water connections. It's not exactly what the regulations call "A local means of isolating all poles of the supply". But what the hell, this is typical of plumbers/kitchen fitters electrical work. Just be careful if you come across any of their jobs.--Test the circuit is off, test your tester and test its off once more.:laugh3:

 

[Darkwood]

Two ways to view this.... if the manufacturers expect the plumbers to connect the electrics then they should pre-flex them issue solved plug it in.... but reality is they are normally flexed on site and all instructions do state to consult an electrician..... that one sentence clears the manuacturers of liability.

 

[RobWansbeck]

Out of interest I've looked at a recent Baxi circuit diagram. This is a 'live chassis' design with only one fuse in the live but a neon between neutral and earth. Since any equipment with a non-polarized plug must have a fuse in each input this can only be to indicate a wiring error.
For years television sets were live chassis and often fitted with 2 pin plugs. The sensible thing to do was to test the polarity before before working on the equipment.

 

[bilge rat]

Out of interest I've looked at a recent Baxi circuit diagram. This is a 'live chassis' design with only one fuse in the live but a neon between neutral and earth. Since any equipment with a non-polarized plug must have a fuse in each input this can only be to indicate a wiring error.
For years television sets were live chassis and often fitted with 2 pin plugs. The sensible thing to do was to test the polarity before before working on the equipment.

Thanks for that. The Baxi concerned was a Baxi neta tec-33 GA. The error on the circuit diagram shows two black wires on the input connections
to the thermostat whilst the actual colours are black and blue. This caused the plumber to connect to the only pair of blacks he could find, a 48volt connection.

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