Thanks. I don't quite understand what the 'special condition' is.
I'm a bit confused here. How does this "subjected to 200 operations" relate to the "tested for 40,000 normal operating cycles" above?
Kind Regards, John
Is it OK(ish) to treat garage MCBs as switches?
- Thread starter skotl
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Kind Regards, John
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If you look at the conditions, the 200 test cycles are not normal operating cycles.
Are you saying that they have to have 40,000 test cycles at rated voltage and current plus an extra 200 test cycles at 1.1 times rated voltage and 1.25 times rated current? If not, what is a 'normal operating cycle'?
Kind Regards, John
Yes.
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That's what I thought, and that was why I attempted to comment (perhaps not clearly enough!) on the dramatic difference between the number of 'normal' and 'abnormal' test cycles - given that the degree of 'abnormality' in the latter is really pretty modest (1.1xV and 1.25xI). However, I presume that people far more clever than myself have decided that such a small number of 'modestly abnormal' test cycles is adequate!
Kind Regards, John
I can't speak for 'we' - but, for myself, I'm sticking with my original intuitive viewpoint, now somewhat supported by the new Note in the regs.
Of course, the chances are that you could probably get way with manually switching an MCB several times a day for decades without doing any harm to it - but it just doesn't seem/feel particularly 'right' to me (particularly given that I can't test MCBs).
Kind Regards, John
Do the new regulations relate to isolation?
Wouldn't that be functional switching?
Isolation would not be required several times a day for decades.
I understand your point. As you imply, the new note to Table 53.4 relates only to functional switching, not to off-load isolation. Mind you, as I explained to stillp, that Table effectively does not distinguish between off-load isolation and on-load isolation - since, apart from fuses, the other 25 devices said to be suitable for isolation are also indicated as suitable for on-load isolation. As you are perhaps implying, the only real difference between 'on-load isolation' (which some would not even call 'isolation') and 'functional switching' is in relation to the perceived probable frequency of operation.
The terminology is obviously a bit messy - are you perhaps saying that you are advocating/supporting the use of MCBs for manual 'off-load isolation' (which is what many mean when they say 'isolation') but not for 'on-load isolation' (which is effectively 'functional switching')?
My intuitive position is simply based on the fact that an MCB switching mechanism is presumably designed primarily to be operated by its 'trip' mechanism, and manual operation really represents a 'brute force over-riding' of that primary intended method of operation. Hence, whether the circuit is broken on-load or off-load, I still feel that it's not a great idea to use that manual 'over-riding' too frequently (whatever that means) whether it is (or one calls it) 'isolation' or 'functional switching'. Whilst I realise that the new note in the regs is in relation to 'functional switching' ('not isolation') I said that it "somewhat supports" my view, since it seems to me that there are at least partially concerned about frequent manual operation (over-riding), whether one calls what one is doing 'switching' or isolation'.
Having said all that, as I have acknowledged, one could probably usually 'get away with' frequent use of an MCB for manual 'functional switching', let alone 'isolation' (however one defines it), for years or decades without any harm coming to the MCB. As I've implied, if I could test MCBs, I'd be far less concerned. However, since I can't, I prefer to leave the 'automatic safety device' just to do what it was primarily designed to do (plus very occasional use for true, off-load, 'isolation').
Kind Regards, John
Isolation has nothing to do with on-load or off-load switching. See http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=826-17-01
It is a state that can be achieved either off-load or on-load, although the devices used to achieve isolation need to be suitable to break the expected currents.
It is a state that can be achieved either off-load or on-load, although the devices used to achieve isolation need to be suitable to break the expected currents.
I realise that. However, its seems from some of the things being written that some people seem to regard 'isolation' (as opposed to functional or emergency switching) as something which achieved by operating an 'isolator' off-load. Indeed, many of the comments being made seem to relate to the difference between on-load and off-load operation of a device.
Once acceptable isolation is achieved, the process whereby it was achieved is clearly of no real relevance.
Kind Regards, John
Functional switching is switching something off, or on, because that's the state you want it to be in. Isolation is separation from source(s) of electrical energy. You can't really compare them.
The intent/purpose of the two activities are obviously very different, but I was talking about the mechanics - particularly in as much as it impacted on, or relied on the capabilities of, the device being used.
In this day and age, I would imagine that (certainly in the domestic situation) most devices capable of (mechanical - semiconductor 'switches' are obviously an exception) functional switching would be designed to also afford an acceptable degree of 'isolation' when in their 'off' state. That is largely reflected in Table 54.3 of BS7671. Most devices said to be OK for functional (or emergency) switching are also said to be OK for 'isolation'. The few exceptions (in addition to semiconductor 'switches') are most of the 'Switching devices' (to 50428, 60669 {-1, -2-1, -2-2, -2-3} & 60947-5-1) and contactors to 61095. Can you tell me what aspects of those render them unsuitable for isolation?
Kind Regards, John
61095, and 60947-5-1 (which is one I work on) have extra tests that are applied when the manufacturer wishes to claim that his device is suitable for isolation. 50428 devices are electronic. 60669-1 doesn't seem to address isolation at all.
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