Sorry for the newbie Q but can someone please explain what the significance of the kA is regarding MCB's? My parents have an old 3 way Wylex CU which is fitted with fuse wire cartridges rated @ 5A (lights),
15A (mains) & 30A (cooker). I was thinking of replacing the fuse wire cartridges with MCB's and looking at screwfix it appears part No 48030 is what I need (6, 16, 32A) which says "Modern alternative to fuses for Wylex Standard Consumer Units. Single pole, 230V AC, B curve, 3kA MCBs. BS EN 60898". I noticed that other types of mcb's are rated much higher at 6kA but is the 3kA for our CU still a safer alternative to using the fuse wire cartridges?
It is a measure of the maximum fault current that the MCB can interupt - kA being Kiloamps or thousands of amps. If the maximum fault current exceeds the rated kA, then in theory the breaker may not open, but be assured that the wiring will !
Yes it is a safer alternative, but they may find that the lighting one trips when bulbs blow, which can be a pain.
That depends... do they get blown fuses often? If so why? If the answer is they only get fuses going occasionally when a bulb blows, then you might argue that while the MCB will be more convenient to reset, but it is also more likely to trip in this case anyway. So the total risk to life and limb may increase since they would need to make more trips to the CU in the dark. If they are getting blown fuses due to overloading, then you really ought to be looking at a rewire to modern standards rather than a quick fix band aid.
The only real safety advantage of an MCB in these circumstances are 1) if there is a danger that someone is likely to rewire a fuse with something other than the correct rating of fuse wire[1], or 2) they are running sustained and substantial overloads on old wiring where the MCB is more likely to trip thus preventing a fire.
The addition of RCD protection for the power circuits if they don't already have it would be a much more useful change. If they have a "whole house" RCD, then addition of some emergency lighting would probably be a bigger safety improvement.
[1] I could not tell from your description if you mean the fuses are cartridges in carriers, or re-wireable fuses.
Longer answer:
The 3kA is the breaking capacity. That tells you the maximum fault current that the MCB will be able to interrupt without the MCB sustaining damage, or worse, failing to interrupt the current.
Is 3kA sufficient? Well that will depend on the specific details of the instalation. Chances are it probably is unless they are in the middle of a big town (and for the pedants it may still be just "ok" even then [2]).
Say for example you introduced a hard short circuit into a cable coming from the CU (e.g. sticking a nail through a power circuit wire). A massive fault current would then flow - and so long as it is less than
3kA the MCB should open rather quickly and disconnect the power. The worse case would be a fault in a circuit very close to the CU.
The magnitude of this fault current (known as the "Prospective Fault Current" (PFC)) will be dictated by the total fault loop impedance upto the point of the fault. With a fault adjacent to the CU this would basically be dictated by the supply impedance itself (i.e the house wiring would not be introducing much extra resistance of its own). To reach a 3kA fault current you would need a supply impedance of 0.08 ohms or less; which is very low. Hence the comment about the property location. Close to a substation, connected via a very big supply cable, this impedance would be possible. However in many real world situations the supply impedance will not be this low and hence the PFC limited.
(for example, I live about 600 yards from a substation as the cable flys, and yet the supply impedance is still getting on for 0.25 ohms limiting my PFC to about 1kA)
[2] In terms of the regulations for breaking capacity it may be acceptable for the MCB to have a lower breaking capacity than the PFC if there is a device on its supply side that is capable of breaking the PFC. In this case you would need to demonstrate that the MCB would not be damaged by the "let-through" energy of the supply side device. Since however the supply side device would be the main service fuse, you would fail to meet the requirements of discrimination instead ;-)
Strictly speaking it's the maximum prospective fault current that the device will interrupt, and the PFC is the RMS current which would flow if the fuse of MCB is replaced with a dead short (solid link). At high fault currents many fuses and (I think) all MCBs are "current limiting"
- i.e. they will interrupt the current before it actually reaches the full prospective value.
All domestic consumer units systems to EN 60439-3 (and its predecessor BS 5486-13) have a conditional fault rating of 16 kA, the condition being the presence of backup protection by means of a 100 A (or less) supplier's fuse to BS 1361 type 2.
The requirement is that discrimination is considered in the design, not that it must always be provided. In reality of course a very high PFC will only exist for a fault in or very close to the consumer unit. It only takes about 5 m of 2.5 mm^2 cable to attenuate the fault level from
16 to to 3 kA, so in practice discrimination will be achieved in the most probable real fault scenarios.
As in "yeah, thought about it, but CBA to design my way round that one" ;-)
Indeed, I expect there are relative few domestic properties where the supply impedance is low enough to make this much of a risk at even 3kA let alone the more typical 6kA.
For most people mcbs are no safer than rewirable fuses. The exception is if you have a nut in the property that might decide to replace fuse wire with something that wont blow, like 2.5mm cable. So replacing them in your own home is not generally worth doing.
Mcbs are prone to nuisance tripping on lighting circuits.
Thanks for the replies, I'm starting to understand a bit more now. It's a bungalow built in the mid 1960's so I'd say is overdue a rewire/upgrade. They do get the odd occasional blow on the lighting circuit but given the amount of 150w bulbs around the home I think they may sometimes be pushing it a bit. At the moment they have the re-wirable fuse type holders which although the old fellow is competent at changing is a PITA. I'll give the MCB a try and see how it goes and make some enquiries into having the whole lot done with a new CU with rcd protection.
if the lighting blows the 5A fuse wire it sounds like they have either a significant overload going on, or else another type of risky fault. Either way an mcb wont resolve the issue - in fact the reduced hassle factor is liable to cause people not to bother rewiring.
Replacing some filaments bulbs with CFLs would cut the power consumption down, and may be all thats needed.
Based on the age there is a fair chance the wiring is rubber insulated. This is almost certain to be well past the end of its safe life. If this is the case you should consider a rewire as a matter of priority.
Remember that a 5A rewireable fuse will need something like 9A or more as a sustained load to actually open the fuse. That would suggest a lighting load of over 2kW... are they really running 14 150W bulbs or their equivalent?
Remember that the RCD is more prone to trip when a bulb blows - so if that is the main source of fuse blows then the frequency is likely to get worse.
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