Fusing the neutral?

[bilge rat]

I recently did a fault finding exercise on a Baxi central heating boiler at my sons house. The plumber who installed it had wrongly connected a wireless programmer. An easy mistake for a plumber to make as the installation manual showed two different versions of the same circuit diagram.
During the check I was horrified to find the connection block within the boiler unit, had both live and neutral conductors fused. Now the last time I worked as an electrician was in the era of the 16th edition. Fusing the neutral was always a no no because of the dangers that could result in the neutral fuse blowing leaving the appliance apparently dead yet still having a live connection to it.
Is it possible that a large company like 'Baxi' could make such an error or have the Regulations changed considerably.
Your views please.

 

[Dan]

Welcome to electriciansforums.net. Stick around I'm sure you'll like the forum.

 

[CRAGS]

Hi and welcome

 

[Top Cat]

Hi bilge rat, welcome to the forum. The first thing you have to realise is that you said a plumber connected it! Look though some of the past posts and you will see that we do not have a lot of time for plumbers!

I will stop now or i could be still on at midnight about f in plumbers!

 

[bilge rat]

I agree about plumbers but the fused neutral is put into the boiler unit by the manufacturer Baxi).

Thanks for the welcome

 

[Top Cat]

I agree about plumbers but the fused neutral is put into the boiler unit by the manufacturer Baxi).

Thanks for the welcome

Funny you should say that. About six months ago i was connecting a boiler and thought, there is a fuse in the neutral on the pcb, thought to myself no, they wouldn't do that i was in a hurry so left it. Now you have me thinking again!

 

[HandySparks]

Got to be worth getting an answer out of Baxi on this one?

 

[bilge rat]

Got to be worth getting an answer out of Baxi on this one?

Thanks for that handy, I have E mailed them but so far no reply.

 

[snowhead]

If you go to the Baxi website and download the install manual for Megaflo 2 system boiler compact, there is indead a fuse in the neutral and live at the incoming terminal block.

There isn't in the 105he tho'

 

[Top Cat]

If you go to the Baxi website and download the install manual for Megaflo 2 system boiler compact, there is indead a fuse in the neutral and live at the incoming terminal block.

There isn't in the 105he tho'

But why?

 

[snowhead]

But why?

I'm not Baxi, I can't answer that, but the fuse is inside an appliance so won't be under the regs.

I see equipment with switched neutrals and permanent lives.

 

[westom]

But why?

Fusing a neutral wire connected to any load is absolutely wrong. Fusing the internal connector that connects to the neutral can be for reasons completely different. For example, a design may support numerous configurations including those that have no neutral connection. Reasons for fusing inside a controller can be for reasons well beyond what an electrician may understand. Without the specific schematics, nobody can say anything more. An engineer who designed that controller probably fused both AC wire lines for specific reasons. Which is electrically different from fusing a neutral wire that feeds that controller.

 

[Des 56]

That's a reasonable answer above, If there was a fuse in the neutral,it would not be for external controls

More than likely there would be alternative neutral connections that were not via the fuse for any external loads

 

[LukeBuckaz]

maybe they fused the neutral 'jut in case' said plumber got the polarity the wrong way round, probably not, probably something much more technical than this!

 

[Darkwood]

most of you are crossing over bounderies here.. its a control board and subject to different rules and regulation to what we are used to, yes agree that fusing a neutral in our installations could kill in the worst case senerio but on a control board this hazard dosn't normally exist (dependent on circuit board design)it will just stop the board working, many boiler manufacturers etc have neutral fusing but it dosn't pose a safety hazard, remember to get their stamp of approval and on to the british market they have to comply to their own reg's... TIN HAT ON!!!.... anyone explain to me what exactly is dangerous about this then i will argue my corner!

 

[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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