Exactly my point - whatever else you do or don't do, the one thing you must not do is isolate just on end of a ring final!
Kind Regards, John
Is it OK(ish) to treat garage MCBs as switches?
- Thread starter skotl
- Start date
Kind Regards, John
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Are you guessing John?
MCBs to IEC (BS EN) 60898 are tested for 4000 cycles of mechanical operation, whilst carrying their rated current.
Yes, sort of (and probably influenced by the Note which has now appeared in BS7671) - hence the 'probably'!
Thanks - that's very interesting. How does that compare with the number of test cycles for other switches and isolators? Four thousand manual cycles sounds like a fair bit - about 11 years at one cycle/day. It makes one wonder about the new BS7671 note ("...they are not intended for frequent load switching"). Does the Standard say anything about whether such a use/practice is 'recommended'? (I suppose that's not really for a Standard to say?).
Kind Regards, John
Are you saying that every MCB is production tested to 6000 cycles ? Probably a number of samples from each batch are so tested before the batch is released to market
That is a bit like the match factory where the quality assurance department insisted on testing every match in every box.
I certainly would be very concerned about a device that had operated under full load 6000 times before it was sold.
Some MCBs in aircraft have to be replaced after they have tripped a few times. Less than 10 if memory serves me correct.
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So if they've tripped more than 10 times they don't have to be replaced?
Exactly. It's not the sort of test one would use for 100% production testing. Indeed, stillp is probably simply telling us what test the Standard specifies the product (type) has to be able to pass - and may well (I don't know for sure) have nothing to say about what sort of production sampling/testing regimes, or statistical criteria, which are necessary to ensure that the product meets that specification.
... exactly the same as with most mass-produced products - from nuts and bolts, through food products to medicines.
Kind Regards, John
4000 Bernard, not 6000. These are type tests, not routine tests, performed to verify the design. They need only be performed once, but it's up to the manufacturer and his QA system to decide how often to repeat the tests, as with most other manufactured products.
Echoes are sometimes reassuring (we were clearly typing simultaneously)
Thanks for confirming what I wrote.Kind Regards, John
Well, 'frequent load switching' might be very much more frequent than once per day. I think the point of the Note is to deter the use of MCBs to replace e.g. light switches for normal use. The standard doesn't make any statements about use, or require the manufacturer to give any instructions for use.
Other products are typically tested for similar numbers of operations, but in some cases that testing is optional - overload relays for example, in the larger sizes, might have an intended operating life of only a few operations. In such cases the manufacturer specifies the rated number of operations.
By the way I hesitate to mention this due to the risk of starting another marathon thread, but the term 'isolator' is deprecated and is not used in standards for switchgear and controlgear. See http://www.electropedia.org/iev/iev...d+field+Language=en&SearchOrder=4&SearchMax=0
The picture someone posted earlier was of a switch-disconnector, which provides the isolation function as well as current switching capability. The red handle with a yellow background indicates that it is suitable for emergency operation.
What used to be known as an off-load isolator should now be called a disconnector. Unfortunately nobody seems to have told the people responsible for wiring accessories, or for 7671.
Fair enough. Thanks.
I can understand that.
Although it may be unavoidable in some situations (e.g. very high current circuits), it's always struck me that the concept of an "off-load isolator", no matter what one calls it, is a little on the iffy side - since, in the absence of interlocks (mechanical, based on current sensors or whatever), it's probably difficult/impossible to guarantee that such a device will never be used to break a load (potentially its maximum rated load). I would therefore have thought that the only safe course would be to require that it could, were it ever asked to, safely break its maximum rated current. ... but that's all just intuition/'common sense', and may not be 'how it is'!
Kind Regards, John
They are often used in enclosed electrical operating areas, which are places like switchrooms where access is limited to electrically skilled persons. An example that will probably horrify you is removable links in busbar systems. Where 'ordinary persons' have access then you're right, disconnectors can be very dangerous and have been known to explode when operated under fault conditions.
That would seem to be a good reason for restricting to 'skilled persons' the use of any sort of disconnector which could not safely break its full rated current. I may be wrong, but I suspect that it may only be a big issue in industrial situations - since I imagine that most 'disconnectors' (probably called 'isolators'
) in domestic and 'light commercial' environments probably are usually able to safely break their full rated current - is that the case?Kind Regards, John
I don't know, I don't deal with domestic wiring accessories, or, as one of my colleagues used to call them, "the toys and fancy goods department"!
What's their Standard number, I'll see if I have a copy.
What's their Standard number, I'll see if I have a copy.
Well, Table 53.4 of BS7671 does not explicitly include any 'disconnectors' or 'switch disconnectors'. It has "Isolating Switch" (BS EN 60669-2-4 and BS EN 60947-3) and some things called "Control and protective switching device for equipment (CPS)" (BS EN 60947-6-1 and BS EN 60947-6-2), all of which it indicates are "suitable for on-load isolation". It also has 8 things called "Switching devices" but, of these, only two (the same two Standards as specified for the "Isolating switches") are specified as suitable for any sort of 'isolation', and both of those are, as above, said to be "suitable for on-load isolation".
In fact, of the 26 things it says are suitable for 'isolation', a fuse is the only one which is not also said to be "suitable for on-load isolation"!
Kind Regards, John
Well, the only things you mention that are likely to appear in a domestic installation are those "Isolating switches" to 60669-2-4. Interesting read that, but difficult as it refers to Part 1 for many items and has alternate pages in French.
Switches for isolation, to give them their correct name, when rated up to 16A, are tested for 40 000 normal operating cycles. There's a Special National Condition: "NOTE In the UNITED KINGDOM, isolators having a rated current below 16 A are widely used and permitted." (Note that they use the deprecated term). They are tested for conditional short-circuit withstand by closing onto a S/C with the recommended SCPD in series. In addition, they are tested as follows:
18.1 Switches are tested at 1,1 times the rated voltage and 1,25 times the rated current.
They are subjected to 200 operations at a uniform rate of:
– 30 operations per minute if the rated current does not exceed 10 A;
– 15 operations per minute if the rated current exceeds 10 A but is less than 25 A;
– 7,5 operations per minute if the rated current is 25 A or more.
Switches for isolation, to give them their correct name, when rated up to 16A, are tested for 40 000 normal operating cycles. There's a Special National Condition: "NOTE In the UNITED KINGDOM, isolators having a rated current below 16 A are widely used and permitted." (Note that they use the deprecated term). They are tested for conditional short-circuit withstand by closing onto a S/C with the recommended SCPD in series. In addition, they are tested as follows:
18.1 Switches are tested at 1,1 times the rated voltage and 1,25 times the rated current.
They are subjected to 200 operations at a uniform rate of:
– 30 operations per minute if the rated current does not exceed 10 A;
– 15 operations per minute if the rated current exceeds 10 A but is less than 25 A;
– 7,5 operations per minute if the rated current is 25 A or more.
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