The following were removed:
- It should be noted that in such an event, MCB or fuse action is intended to much reduce the risk of fire, but it does not prevent it, and a repeatedly tripping mcb is not a safe situation.
- Note that neither fuses nor MCBs are designed to act on all fault currents, and failure to trip does not necessarily indicate a fault with the MCB or fuse.
Imho these are 2 significant points for anyone seeking to understand the action of fuses & mcbs. NT 14:50, 6 January 2011 (UTC)
Keep in mind that this section is supposed to be a comparison between fuses and MCBs.
- "It should be noted that in such an event, MCB or fuse action is intended to much reduce the risk of fire, but it does not prevent it, and a repeatedly tripping mcb is not a safe situation"
Part of it seems to apply equally to both, and so is not really a comparison. The salient point, that there is a possibility of abuse by repeated resetting is still there, and seems more fitting to something specific to MCBs and not as applicable to fuses.
- "Note that neither fuses nor MCBs are designed to act on all fault currents"
That bit as written is obviously wrong - probably just due to sloppy terminology. But fuses/MCBs are most definitely required to respond to all fault currents. (cf. they may not act on all overload currents)
- "and failure to trip does not necessarily indicate a fault with the CB or fuse. "
In the case of a failure to act on a fault (i.e. short circuit) then that is either a failure of the device, or of the circuit design (or possibly a failure of the installation earth).
I suspect that the original intention of the last bullet was to highlight that the nominal rating of the device is not the same as its tripping current. This is a valid point and perhaps ought to be included in the intro for both fuse and MCB articles (or just reference the section in the calculating cable size article). Again however the point applies equally to a fuse or MCB and hence does not really seem appropriate in a section attempting to spell out the differences.
--John Rumm 16:04, 6 January 2011 (UTC)
Looks to me like there's some misunderstanding. I'll take this to the newsgroup so we can explore, and see whre to put any reuslting material. NT 21:13, 6 January 2011 (UTC)
I am not sure what needs to be discussed.
Reg 411.4.4 The following types of protective device may be used for fault protection (i) An overcurrent device (ii) An RCD (but that also needs to work with an overcurrent device)
So we use a fuse or MCB for fault protection with additional RCD potection against earth faults when needed.
--ARWadsworth 21:40, 6 January 2011 (UTC)
Breaking capacity of older MCBs
I am sure that they had a M rating. So the M1 was a 1kA device. I'll check on this one.
Re breaking capacity, I'm fairy sure the following is not correct:
Circuit designers may need to provide additional "back up" breaking capacity to ensure adequate protection when designing for systems with these very low fault impedances.
AIUI the incomer fuse does the job when the mcb fails to. It just means a high current short is messier, and takes the lot out. Inconvenient, but not a danger. NT 17:39, 10 September 2012 (BST)