Electric shower isolation

[John D v2.0]

I suppose so, in some senses, but 'pulling fuses' under load is primarily a hazard to the person trying to do it, rather than a reflection of the inability of the device to 'break' its rated current. To put it another way, 'breaking capacity' is not really a characteristic (or intended use) of fuses.

Kind Regards, John

I wouldn't look at it that way, it doesn't seem necessary.

• a fuse carrier is an isolator, as the fuse can be removed and the carrier replaced and sealed
• the definition of breaking capacity is the maximum current a device is designed to open.
• a fuse cannot be pulled with any load, therefore the breaking capacity is zero.

This isn't like a "divide by zero" situation, it's just logical. Although I'll admit that makers will describe that situation as "do not pull under load" rather than "breaking capacity zero"

(for completness, of course the fuse itself itself has a fault breaking capacity, but we're not talking about that mode of operation)

 

[JohnW2]

I wouldn't look at it that way, it doesn't seem necessary. .... a fuse carrier is an isolator, as the fuse can be removed and the carrier replaced and sealed ...

We're probably back to 'word issues' again. A fuse is certainly capable of effecting 'isolation', at least in a TN installation (**), but I don't think many people would call it 'an isolator'. [** At least as far as BS7671 is concerned, a fuse could not achieve 'isolation' in a TT installation, since it requires DP isolation in such an installation - and, of course, neutral fusing is also 'not allowed.]

... the definition of breaking capacity is the maximum current a device is designed to open....

I would say that it's the maximum current that a device can 'safely' break, whether it was 'designed to do so' or not. However, I will agree that it is not intended to break any current (by being removed, rather than by 'blowing') - although that's probably more a matter of the design of the fuse holder than of the fuse itself.

Given the unpredictability as to what users will do with them, I would imagine that the relevant standards probably require plugs/sockets (which are about the best means of 'isolation' possible) to be capable of fairly safely breaking the maximum current they are designed to carry, but I doubt that anyone would advocate that practice, or would refer to them as isolators.

I merely made the observation (with which you agreed) that almost everything now sold as 'an isolator' is rated to be able to safely break its rated carrying current, and I don't think that bringing fuses (never sold as 'isolators') into the equation helps the discussion much

Kind Regards, John

 

[John D v2.0]

fair enough, plugs are pretty good isolators too but not described as such, and can make/break 13A, but there can be pin damage on some laptop chargers I've noticed.

 

[JohnW2]

fair enough, plugs are pretty good isolators too but not described as such ....

Indeed - that's more-or-less exactly what I said.

... and can make/break 13A ....

As I said, I imagine that they are required to be able to do so fairly 'safely', but I would also imagine that manufactures would probably be hesitant to say that they 'can' do that, since they would thereby appear to be supporting something which I suspect most people would regard as an undesirable practice.

... but there can be pin damage on some laptop chargers I've noticed.

Are you talking about the BS1363 plug on the end of the cable? If so, I would have thought that breaking a 2kW or 3kW load would be likely to do a lot more damage than a laptop charger.

Kind Regards, John

 

[John D v2.0]

Are you talking about the BS1363 plug on the end of the cable? If so, I would have thought that breaking a 2kW or 3kW load would be likely to do a lot more damage than a laptop charger.

not sure why, but laptop charger pins don't seem to be always made from best quality materials!

 

[JohnW2]

not sure why, but laptop charger pins don't seem to be always made from best quality materials!

Ah!

Kind Regards, John

 

[EFLImpudence]

Table 53.4 would seem to disagree with you both about what are isolators and emergency switches.

 

[John D v2.0]

A fuse is certainly capable of effecting 'isolation',

plugs/sockets (which are about the best means of 'isolation' possible)

a fuse carrier is an isolator, as the fuse can be removed and the carrier replaced and sealed

plugs are pretty good isolators too but not described as such

Table 53.4 would seem to disagree with you both

assuming JohnW and I were each of the "both", the relevant information seems to support our understanding about what was an isolator

 

[JohnW2]

Table 53.4 would seem to disagree with you both about what are isolators and emergency switches.

As I said, there is a difference between a device which is suitable to provide isolation and 'an isolator', and I don't think that many people would dream of describing a fuse as 'an isolator'.

We've discussed before the fact that Table 53.4 now says that 'isolating switches' are suitable for 'emergency switching' (hence capable of breaking a full load current), which presumably means that the relevant Standards have changed such that 'traditional isolators' not designed to break current probably no longer exist - hence my comment that it seems that any 'isolator' one can now get will also be rated to break a current equal to its full CCC.

I see that the Table also says that BS88 and BS1362 fuses and unswitched FCUs ("by removal of fuse") are acceptable for 'isolation' - but, as I said previously, one cannot get BS7671-compliant isolation in a TT installation by the use of a fuse.

Were there other observations about the Table that you wanted to make?

Kind Regards, John
Edit: Somewhat slow typing again!

 

[EFLImpudence]

From reading your posts, I did not get the impression that you agreed with Table 53.4.

a fuse cannot be pulled with any load, therefore the breaking capacity is zero.

As I said, there is a difference between a device which is suitable to provide isolation and 'an isolator', and I don't think that many people would dream of describing a fuse as 'an isolator'.

Note 3 of Table 53.4 says those devices are suitable for on load isolation, and
note 4 deals with TT.

 

[JohnW2]

From reading your posts, I did not get the impression that you agreed with Table 53.4. .... Note 3 of Table 53.4 says those devices are suitable for on load isolation ....

Not for fuses (BS88 or BS1362) - so, in keeping with common sense, they are saying that fuses are not suitable for on-load isolation.

I'm not actually sure what they mean when they say that BS1362 fuses are suitable for isolation, although not on-load isolation. Do they simply mean removing the fuse from a plug, and then plugging it into a socket - but who would be daft enough to do that?! (they deal with FCUs separately).

... and note 4 deals with TT.

Indeed, confirming what I said.

Kind Regards, John

 

[ericmark]

Rules do seem odd, I was told by social services to stop my mother going into the kitchen where there were knives and hot surfaces was against her human rights, unless sectioned I could not stop her. Yet when she went into a care home, their kitchen was locked away from the residents.

Back to electrical yes emergency switching and isolation are very different. But I think the question has been answered, you don't "need" local emergency switch or isolator, but it is in the main desirable to have it.

 

[John D v2.0]

I'm not actually sure what they mean when they say that BS1362 fuses are suitable for isolation, although not on-load isolation. Do they simply mean removing the fuse from a plug, and then plugging it into a socket - but who would be daft enough to do that?!

They simply man that a plug without a fuse provides effective isolation.
Daft it may be, but to be honest I thought it was perfectly normal and sensible to remove a fuse to isolate something that can be plugged in. I usually do it before throwing away a cut off moulded plug and lead for any reason.

 

[ban-all-sheds]

Rules do seem odd, I was told by social services to stop my mother going into the kitchen where there were knives and hot surfaces was against her human rights, unless sectioned I could not stop her. Yet when she went into a care home, their kitchen was locked away from the residents.

It wasn't her kitchen.

She had no more of a "human right" to go into it than you or I have to go into the kitchen (or office) of a hotel or a restaurant where we happen to be staying/dining.

 

[ban-all-sheds]

As I said, there is a difference between a device which is suitable to provide isolation and 'an isolator', and I don't think that many people would dream of describing a fuse as 'an isolator'.

What about a fuse switch?

Although whether you get any with BS1362 fuses in, I don't know - I'm not aware that they are used anywhere except in plugs, FCUs, multi-way adapters and conversion sockets. I'd not be surprised if there were panel, or PCB, fuse holders made which used them, because of their ubiquity, but they would be out of scope.

We've discussed before the fact that Table 53.4 now says that 'isolating switches' are suitable for 'emergency switching' (hence capable of breaking a full load current), which presumably means that the relevant Standards have changed such that 'traditional isolators' not designed to break current probably no longer exist - hence my comment that it seems that any 'isolator' one can now get will also be rated to break a current equal to its full CCC.

No - see Note 1.