DP fusing

[plugwash]

DP fusing has it's pros and cons, the obvious pro is it can limit the current flow though a neutral to earth fault. It can also provide protection if live and neutral have been swapped somewhere in the system.

The con of course is if the neutral fuse does blow it can leave the load in an "off but live" state.

 

[JohnW2]

DP fusing has it's pros and cons, the obvious pro is it can limit the current flow though a neutral to earth fault.

It might sound 'obvious', but is it ever going to happen? Unless there were (extremely unlikely) simultaneously a supply-side fault which raised the potential of neutral to an abnormal level, you surely would never get enough current due to a N-E fault to blow a neutral fuse?

In fact, it would seem impossible to get a significant N-E current with a single N-E fault in a TN-C-S installation, whilst very few TT electrodes would ever carry enough current to blow a fuse of 5A or more (even if there were a simutaneous supply-side fault). That seems to only leave (very unlikley) the situation of simultaneous supply-side fault and local N-E fault in a TN-S installation.

Kind Regards, John.

 

[plugwash]

That seems to only leave (very unlikley) the situation of simultaneous supply-side fault and local N-E fault in a TN-S installation.

Given how NE faults can sit unnoticed for years it doesn't seem all that unreasonable for a lost (or just poor) neutral somewhere to force a LOT of current down a N-E fault in a TNS system. Especially in a time before RCDs. Not hugely likely but potentially extremely damaging if/when it does happen.

Even in a TT system it could happen if a supply tail fell out between the seperation point and the CU.

 

[JohnW2]

That seems to only leave (very unlikley) the situation of simultaneous supply-side fault and local N-E fault in a TN-S installation.

Given how NE faults can sit unnoticed for years it doesn't seem all that unreasonable for a lost (or just poor) neutral somewhere to force a LOT of current down a N-E fault in a TNS system. Especially in a time before RCDs. Not hugely likely but potentially extremely damaging if/when it does happen.

Yes, as I said, that is the one theoretical possibility - but I presume that most people would agree that such a (pretty rare) possibility does not outweigh the dangers presented by neutral fusing.

Even in a TT system it could happen if a supply tail fell out between the seperation point and the CU.

I don't really understand that. Even if the neutral potential were to rise to 250V relative to earth, given that the Ze of domestic earth rods is not likely to be much lower than 50, even a dead short between N & E would not result in enough neutral current to blow a 5A fuse.

Kind Regards, John.

 

[plugwash]

That seems to only leave (very unlikley) the situation of simultaneous supply-side fault and local N-E fault in a TN-S installation.

Given how NE faults can sit unnoticed for years it doesn't seem all that unreasonable for a lost (or just poor) neutral somewhere to force a LOT of current down a N-E fault in a TNS system. Especially in a time before RCDs. Not hugely likely but potentially extremely damaging if/when it does happen.

Yes, as I said, that is the one theoretical possibility - but I presume that most people would agree that such a (pretty rare) possibility does not outweigh the dangers presented by neutral fusing.

Indeed, with our modern supply and wiring situation in this country the risks presented by neutral fusing outweigh the benefits which is why we don't do it at least in fixed wiring (I belive some portable applicased use fused netruals because they are also sold to places that have unpolarised sockets).

Of course double pole common trip breakers give the best of both worlds.

Even in a TT system it could happen if a supply tail fell out between the seperation point and the CU.

I don't really understand that. Even if the neutral potential were to rise to 250V relative to earth, given that the Ze of domestic earth rods is not likely to be much lower than 50, even a dead short between N & E would not result in enough neutral current to blow a 5A fuse.

Sorry that bit should have said "even in a TN-C-S system".

 

[Guitarguy]

Double fused cut-outs were banned in the 1930's!

Ah, the very man Yes, I know they were banned in the 1930's, but I also know (as you do) that we haven't yet been able to total rid ourselves of them 70-80 years later!

Anyway, maybe you know what the thinking was that led to their existance before they were banned?

Kind Regards, John.

Me and my work colleague changed an old cast iron cutout a couple of months ago and it had re-wireable fuses on pole the live and the neutral. It was so old we think the service cable feeding it might of been jute (jute string insulation soaked in oil rather than papers) No one really knows they are there until you go to change them or work on them etc. Alot of the ancient cast iron cutouts i come across have a henley block in place on the neutral instead of the small cast iron fuse holder that would of originally been there.

 

[JohnW2]

Indeed, with our modern supply and wiring situation in this country the risks presented by neutral fusing outweigh the benefits which is why we don't do it at least in fixed wiring

Quite so.

Of course double pole common trip breakers give the best of both worlds.

They do indeed, and they're easy enough to make. However, as the regs move closer to requiring every circuit to have RCD (as wellas OPD) protection, I wonder if they would actually offer anything. I haven't thought this through too deeply, so may be missing something, but wouldn't an RCD catch most, if not all, of the scenarios we've been talking about?

Even in a TT system it could happen if a supply tail fell out between the seperation point and the CU.

I don't really understand that.....

Sorry that bit should have said "even in a TN-C-S system".

Fair enough.

Kind Regards, John.

 

[JohnW2]

Alot of the ancient cast iron cutouts i come across have a henley block in place on the neutral instead of the small cast iron fuse holder that would of originally been there.

That has persisted beyond the days of cast iron, as in this picture of my 'suply head' which I've posted here before (the incoming neutral kindly identified by the red inner insulation which you can see ):

Untitled

• JohnW2
• 13 Aug 2011
• 5

Kind Regards, John.

 

[Guitarguy]

Alot of the ancient cast iron cutouts i come across have a henley block in place on the neutral instead of the small cast iron fuse holder that would of originally been there.

That has persisted beyond the days of cast iron, as in this picture of my 'suply head' which I've posted here before (the incoming neutral kindly identified by the red inner insulation which you can see ):

Untitled

• JohnW2
• 13 Aug 2011
• 5

Kind Regards, John.

Is that an overhead supply?

If so i guess they just used the same coloured lead in's from the connection outside on the wall or wherever it is....

 

[JohnW2]

Is that an overhead supply? If so i guess they just used the same coloured lead in's from the connection outside on the wall or wherever it is....

Yes, it's an overhead supply; the singles you see go quite a long way along an outside wall up to where they connect to the ABC which goes to the pole. When I've raised the question (of the insulation colour) here before, it's been suggested that 'they' probably only carry cable of one colour in their vans, to save space, so use it for everything

Kidn Regards, John.

 

[plugwash]

Indeed, with our modern supply and wiring situation in this country the risks presented by neutral fusing outweigh the benefits which is why we don't do it at least in fixed wiring

Quite so.

Of course double pole common trip breakers give the best of both worlds.

They do indeed, and they're easy enough to make. However, as the regs move closer to requiring every circuit to have RCD (as wellas OPD) protection, I wonder if they would actually offer anything. I haven't thought this through too deeply, so may be missing something, but wouldn't an RCD catch most, if not all, of the scenarios we've been talking about?

A proper RCD with double pole switching would indeed.

You have to be careful with RCBOs though. Afaict pretty much all single module RCBOs have a "solid neutral". So while they will detect a neutral to earth fault they can't disconnect it.

 

[JohnW2]

I haven't thought this through too deeply, so may be missing something, but wouldn't an RCD catch most, if not all, of the scenarios we've been talking about?

A proper RCD with double pole switching would indeed.

Yes, and that's obviously what meant (are there such things as SP RCDs?). A SP device (such as a SP RCBO) would clearly not do the job, but DP RCBOs seem fairly rare, and pretty expensive.

Kind Regards, John.

 

[SUNRAY]

Double fused cut-outs were banned in the 1930's!

but still fitted in the 50's
and still in use till I asked for an isolation in the 00's
when the N fuse was replaced with a link

 

[SimonH2]

In the case of my grandmother, who didn't die until 1989, that 'respect' manifested itself as total fear ...
...
her husband (my grandfather) would happily connect anything (heaters included) to a bayonet plug and plug it into a lampholder, have amazing Christmas trees of adaptors eminanting from sockets and lampholders, would join cables by twisting together the live conductors and leaving them exposed ...

Do you not think the two might be related


One thing that does come to mind about the original comment that started this. As others have said, when some of these bits of kit were designed, electrical supplies etc weren't as standard as we are used to - and I believe there were a lot of reversible connectors about. So DP fusing probably made sense.

Also, pretty well valve equipment has high voltages in it, and many have exposed chunks of live metalwork if you get the covers off. It's certainly common for both sides of the speaker to be live. Thus anyone taking the covers off a box of valves would be expecting to meet dangerous voltages - and therefore would be taught (either by education or experience) to follow safe working practices. Thus having a blown neutral fuse might not be as dangerous as it would be today - when many people are accustomed to being about to poke around a working box because it's all ELV
As an aside to that, a friend is a radio amateur and I get to read his RadCom (RSGB newletter) - not long ago they had a series of articles about making DC power supplies, in the "several amps, many kV" sort of range Needless to say, a significant amount of the design was around how to make them "reasonably" safe.