PSC higher than MCB breaking capacity

Hi all,

Looking for a practical answer here;

I've read a couple of threads on this but to no clear solution:

  • MCB rated at 3kA breaking capacity
  • 7kA recorded
  • Upstream is a BS88 315A fuse

434.5.2 effectively says something around you should consider the supply side protective devices without giving any further advice (that I understand at least)

After reading discussions on here it seems that the downstream MCB will only be damaged if it tries to operate whilst the fault is present.

Am I correct that to prove this I need to find the worst case operating times at 3kA for the fuses and that should be bigger than the best case operating time for the MCB for this to be safe?


  • The upstream BS88 will be enough for the fault current protection for the installation. It would be better to get a 7kA rated breaker, but, in the worst case, the breaker will be damaged, but, would it really be the worst case that does happen?

    There is added resistance from the source to the end of the final circuit. To get that fault current (and how has it been tested, MFT's are not reliable at higher currents) the fault would have to be at the source. On your final circuit, the impedance will reduce the fault current somewhat, and may even comply at the far end. 

    If it bothers you that much, and is likely to cause more than an inconvenience, an easy solution is to add a fuse (BS88 - 16kA+) in an enclosure,and run the final circuit through that, but I think that is going too far, your upstream BS88 is enough for most people.

  • Hi Alan, thank you for your response.

    We probably have at least 50 of these installations on site, so I need to either say we replace for breakers with appropriate ratings or prove why "The upstream BS88 will be enough for the fault current protection for the installation." 

    How can I prove this?

  • Hi Alan, thank you for your response.

    We probably have at least 50 of these installations on site, so I need to either say we replace for breakers with appropriate ratings or prove why "The upstream BS88 will be enough for the fault current protection for the installation." 

    How can I prove this?

  • I am surprised that what sounds like an industrial installation has MCBs with such a low breaking capacity, but perhaps they are old.

    You need to ensure that the PFC is high enough at the extremity of a circuit to trip in the event of a fault there, but low enough to ensure that the protective devices are not damaged by a fault at the supply end.

    It may be that the let through energy of the fuse will not be high enough to damage the MCBs, but I shall leave it to those cleverer than I to discuss that.

    In the mean time, it sounds as if the installation does not comply with 432. Note that if the MCBs were changed for 10 kA ones, and if the CPCs have been selected by calculation, you would need to ensure that they still comply because I²t is likely to be higher.

  • We do need to know some more details about the upstream fuse. You say BS88, but there are many parts to that standard. We need to know which part of BS88 the fuse complies with. The issue is also likely to affect a fault in the first few meters of cable, as its resistance will lower the fault current. That said, 10KA mcbs are readily available. 



  • let though energy - the 'I squared t' or joules per ohm, is the makers info that you need to know to see what might happen in what order.

    In the event of a silver stake fault near the origin (rare) either you must accept that the fuse will race the breaker and  may operate  betore the breaker. If this happens, is it acceptable in your case- I presume more than one thing loses power if the fuse fails ?

    If you can get the I2t data on the breakers and fuse, I, and I'm sure many others, would be happy to walk through the calculation with you. typical fuse data  shows the pre-arc and let-through in that case as a 'bull-rush' graph, but it may be tables (page 6 of the PDF) . Any current current time product event as a lower rated fuse or breaker blows  that ends up below the "head" of the bull-rush will leave the fuse intact, any event involving more current or more time than the top of the head will always  be cut-off early by the fuse opening. Any event in the middle is undetermined.

    Even if it comes out on the wrong side, and the MCB could be  damaged at the energy  needed to ensure the fuse blows, this may be accepted, as such a fault will be a rare event, and the breaker is presumably in an enclosure that would catch any bits that fell off,  but you may need a spare breaker !!

    A 3kA breaker has been tested is you can be sure it can interrupt a 3000A fault current and live to fight another day, With more current it may work fine, but it may break and stay broken, or it may get hot enough at the contacts to weld up in the on position, (and in the extreme case of welding and no suitable upstream energy limiting device it may split open and emit plumes of fire and molten metal... ). In either of the latter cases it needs changing, but so long as something else cuts the power off in that case,  the outcome is not necessarily dangerous - depending what else happens as a consequence .

    The fuse does not limit the current like a resistor would but it does cut it off mid-cycle, reducing the energy dissipated and so reducing the heat damage to the breaker, and anything else  getting hot in the fault path.


    edit, to give numbers to conjure with that are the right sort of size, an I2t of a million joules per ohm is 1000A for a second, or about 3000A for 1/3 of a second, or 10kA for 1/100 of a second or any similar combination.

    edit to edit, A way to think of  that million joules per ohm means that every ohm in the fault loop gets heated by 1 megajoule - about a single stick of dynamite worth of bang, or the energy in a large mars bar. Do  not confuse the two, the speed of energy release is a big factor here, you do not eat a mars bar adiabatically - that is to say there is time for the energy to spread about the body and be dissipated as it is slowly released.. A kilo joule or two in contrast is the energy in completely burning one smokers match, approx. (reference)

    Now that figure would be  plain bonkers on an LV system as if you really had a whole ohm in series the fault current would have to  be less than 250 amps, but you can still consider that some grubby contacts with a more credible resistance of say 0.01 ohm could get a heat damage equivalent to 1% of a stick of dynamite, more like a small box of matches going off,  and as such you might still not like it.

  • Well if you were to be faced with this question in your City and Guilds 2391 multiple choice exam a) C1 b) C2

    c) C3 d) FI and you chose anything other than answer b) the computer would mark you wrong! So if you were lucky enough to have an excellent tutor like Mapj, I imagine you would plump for answer d). 

  • Gut feel is that 315A fuse is rather large to be directly feeding 3kA MCBs - it would be much more usual to have a lower rating (say 100A or less) in between (what's the rating of the boards the MCBs are in?)

    A 3kA MCB and a 100A fuse with a PFC of 7kA wouldn't worry me greatly - most consumer units are rated for 16kA when supplied by a 100A BS 88-3/BS 1361 fuse after all, even though that's well above the breaking capacity of the MCBs themselves.

    As for proof, you'd likely need the MCB manufacturer's data - they usually quote suitable backup fuse sizes for varying fault currents.

       - Andy.

  • I wonder where the 7KA came from. I doubt you could have confidence in any of the standard MFTs giving an accurate reading at those relatively  low levels of impedance (resistance really).

  • Ah  it is more complex.

    If (and only if) there is a silver stake fault very close to the MCB, so that there is a fault current greater than 3kA it is possible that the MCB may fail. Now without info we do not usually have, we do not know how,  - maybe  it will work anyway,  maybe it will open and never close again, maybe it will weld shut and never open, or maybe great jets of flame will emerge from the sides and seek out anyone nearby with a beard. We just do not know, just it is out of spec. But in a suitable enclosure, all of these possible outcomes may be  contained, and considered as "safe" potential outcomes (though some are inconvenient as you need a new MCB) so long as the fuse operates promptly before anything else, like say the wiring, catches light.

    Now that is not the normal adiabatic limit, that is about damaging PVC insulation so the cable life is shortened, but actually the real upper bound of instant cable damage, you can accept if you do not mind replacing the cable, is much higher,

    Now really you should look at the fuse data and what they do in the range 3000-7000A. I imagine that there will be a fairily constant I2t, but check.

    This represents the worst let through energy that the cables downstream may see - even if the MCB failed to open at all, and can be compared with cable ratings.

    The MCB I suspect stops getting faster at higher fault currents due to the mechanical limits of moving contacts and arc extinction, so unlike the fuse  the let-through energy rises ever more with increasing fault current.

    Then you need to consider which cables are credibly at risk more than a few tens of milliohms down the wire and the fault current is much reduced. If the first few metres are big chunky submains to other boards, and these are unlikely to be damaged while the fuse blows, that is OK.

    really thin stuff like lighring circuits you may decide woud be replaced anyway if accidentally drilled and accept that if the cable is 'lifed' during the last desperate act of blowing the fuse it is cheap enough to replace and you do not care.

    Or you may decide you really do care about all these things, and the consequences of the fuse doing the heavy lifting, ever, are just not acceptable in this case, and then you change the MCB for a large one, or fuse down into sub-circuits where the loss of any one is not such an issue.


    edit curse this forum - I have already answered this question and did so again as I did not see it.