20kA ipf on 15A 3036 fuse

[Aaron b]

Hello, I changed a socket over earlier and tested the circuit after. The fault current was 20kA, Am I right in thinking I only need to consider the breaking capacity of the service fuse that's likely to be 80kA? I'm struggling to find any reference material.

 

[Vortigern]

Presumably you had the ATT function off in doing the Ze test was it?. Anyway, if you read the manual on your MFT you will find that the figure you are looking at could be 6 digits up or down from what you are reading. So you could have 0.07 which could be seen as 0.1 which then means your PFC is 2.3ka (to 3.3Ka) Usually a type II main fuse (old) 1361 is rated at 33Ka Type I 16Ka so depends on type, and yes it may be counted as safe by regs. Of course your original figure would far surpass the Icn of the MCB no doubt. Actually I suppose that is another way of saying what @pc1966 said.....

 

[UKMeterman]

Presumably you had the ATT function off in doing the Ze test was it?. Anyway, if you read the manual on your MFT you will find that the figure you are looking at could be 6 digits up or down from what you are reading. So you could have 0.07 which could be seen as 0.1 which then means your PFC is 2.3k. Usually a type II main fuse (old) 1361 is rated at 33Ka Type I 16Ka so depends on type, and yes it may be counted as safe by regs. Of course your original figure would far surpass the Icn of the MCB no doubt.

What MCB was being discussed?

 

[Vortigern]

What MCB was being discussed?

I am taking a big leap here and assuming there will be a circuit in a consumer unit that houses an MCB which will no doubt be subject to the fault. I assume if there was not an MCB then the PFC would not really mattter as there is no circuit to cause a fault. Could be wrong though...I do see what you are referring to though as the 3036 being mentioned but even that could not withstand a fault of that level.

 

[UKMeterman]

I am taking a big leap here and assuming there will be a circuit in a consumer unit that houses an MCB which will no doubt be subject to the fault. I assume if there was not an MCB then the PFC would not really mattter as there is no circuit to cause a fault. Could be wrong though...

Hi,
Look up 3036 fuse in the regs.

 

[Vortigern]

Why dont you just tell me what you are getting at, can't be bothered to do so right now, bit tired and having a relax? Could you be referring to 2Ka, whats on your mind?

 

[UKMeterman]

We are discussion the shortcomings of the old Wylex rewireable fuses in old wooden backed consumer units that should have been pensioned off years ago. No MCB in sight. They are known for very low breaking capacity.

 

[Vortigern]

Well I understand you were discussing that, however I introduced a slightly different tack to increase the subjects parameters. I hope that has not ruffled any feathers? And anyway the OP was asking about reference material pertinent to the matter raised. I am hard pressed to see he was concerned about the obsolescence of 3036. I was trying to give a slightly more contextual basis for interpreting results in such a scenario.

 

[Aaron b]

Well I understand you were discussing that, however I introduced a slightly different tack to increase the subjects parameters. I hope that has not ruffled any feathers? And anyway the OP was asking about reference material pertinent to the matter raised. I am hard pressed to see he was concerned about the obsolescence of 3036. I was trying to give a slightly more contextual basis for interpreting results in such a scenario.

Thanks for your post.

Well I understand you were discussing that, however I introduced a slightly different tack to increase the subjects parameters. I hope that has not ruffled any feathers? And anyway the OP was asking about reference material pertinent to the matter raised. I am hard pressed to see he was concerned about the obsolescence of 3036. I was trying to give a slightly more contextual basis for interpreting results in such a scenario.

Thanks for your post. Although it's probably now not a problem it sounds like I'm not completely crazy for thinking the fault clearing can be provided my the service fuse. In the scenario I painted above I see that it wouldn't be a good idea.

I'll do an eicr before dealing with the fuseboard.

I wonder if it's because of the rating of the 1361 fuse that the mcbs are rated to break 16kA...

 

[Vortigern]

The cosumer unit is rated to 16Ka not the mcb. Usually the MCB is rated for 6Ka in domestic and 10Ka in commercial. In the regs it does say if the upline fuse is rated to a higher Ka then it is permissible to allow a lower Ka on the MCB to stand. The problem with the 3036 fuses is that if a fault that is higher than the capacity of the fuse occurs then there are molten bits that fly everywhere and a real meltdown of the fuse carrier which can cause fire and expose live parts, especially when the front cover is missing. It would be advisable to look at BPG4 on the electrical safety .org site regards these particular fuse boxes so you don't fall foul of trading standards in saying it needs to be replaced.

 

[UKMeterman]

Thanks for your post.

Thanks for your post. Although it's probably now not a problem it sounds like I'm not completely crazy for thinking the fault clearing can be provided my the service fuse. In the scenario I painted above I see that it wouldn't be a good idea.

I'll do an eicr before dealing with the fuseboard.

I wonder if it's because of the rating of the 1361 fuse that the mcbs are rated to break 16kA...

Hi, this might help, https://www.eca.co.uk/CMSPages/GetFile.aspx?guid=70c38dba-827c-4f7c-ac5b-a29819b5ef7e the consumer unit is type tested so that the assembly can cope with 16KA faults IF there is the specfied fuse type up stream. The thinking is that the fuse will provide some fault limiting by melting very quickly before the current has reached a high level (inductance of cables becoming significant and so on)

 

[Aaron b]

The cosumer unit is rated to 16Ka not the mcb. Usually the MCB is rated for 6Ka in domestic and 10Ka in commercial. In the regs it does say if the upline fuse is rated to a higher Ka then it is permissible to allow a lower Ka on the MCB to stand. The problem with the 3036 fuses is that if a fault that is higher than the capacity of the fuse occurs then there are molten bits that fly everywhere and a real meltdown of the fuse carrier which can cause fire and expose live parts, especially when the front cover is missing. It would be advisable to look at BPG4 on the electrical safety .org site regards these particular fuse boxes so you don't fall foul of trading standards in saying it needs to be replaced.

Thats what I was thinking of, it's the exception to 434.5.1.

Is that the MCB's would be fubar'd over 6kA though but would break up to 16kA?

There are regularly extension leads posted through the window so RCD protection is going to be required. The fuse box is currently under a kitchen cupboard and with the occupiers being elderly it's not overly practical. It has become a bit confusing with quite a few sRCD's scattered around and a few garage units, they want a few additions so sorting it out would be worthwhile.

I used to be quite concerned by wooden backs on fuse boards but seeing that it's hardwood that doesn't burn too easily and considering the issues with the plastic ones I'm more relaxed. I don't really buy the often spouted comment about the regs not being retrospective, that could only apply where the usage hadn't changed.

 

[Aaron b]

Hi, this might help, https://www.eca.co.uk/CMSPages/GetFile.aspx?guid=70c38dba-827c-4f7c-ac5b-a29819b5ef7e the consumer unit is type tested so that the assembly can cope with 16KA faults IF there is the specfied fuse type up stream. The thinking is that the fuse will provide some fault limiting by melting very quickly before the current has reached a high level (inductance of cables becoming significant and so on)

Thanks, it sounds like if this was a concern I wouldn't need to worry about the let-through energy of the BS88 fuse as it's designed for it.

 

[pc1966]

Is that the MCB's would be fubar'd over 6kA though but would break up to 16kA?

Most MCBs have two ratings, one is the once-only break (Icu), the other is the break and survive to serve another day rating (Ics). If you exceed the once only rating chances are it will explode, not a good situation.

However, when you have more than once OCPD in cascade you sometimes get a higher rating depending on the break time constants and I2t limiting behaviour of them. If you take the common case in the UK of a BS88 fuse up-stream of a typical domestic DB then the fuse has this sort of a current-limiting characteristic:

If you look along the bottom X-axis for around 16kA RMS PFC and then go up to the plot for a 100A fuse you see it limits the peak current to around 10kA, then if you follow that back to the unlimited line and down you see that is equivalent to the peak of a symmetric fault of around 4.5kA RMS, so your 6kA MCB ought to survive OK.

Also remember it takes very little resistance to bring the current down, 16kA implies Zs is around 0.014 ohm, 6kA would be 0.038 ohm so 0.024 ohms more, with 2.5mm T&E at 19.5 mOhm/m for R1+R2 it is about 1.2m of cable.

 

[pc1966]

If you consult the manufacturer's data for cascading values you will see what is OK or not, and in some cases it can be surprising.

For example, Hager (only reason here is I am familiar with them) give the breaking limit for for MCB and BS88 ranging from 6kA (commercial MCB when the MCB is having to break majority of energy on 100A fuse) to 80kA (i.e. the fuse limit when cascade is working well), and from 18kA to 50kA for industrial MCB/MCCB cascades, depending on the upstream MCCB (but not reaching the 70kA upper MCCB limit).

 

[Aaron b]

Most MCBs have two ratings, one is the once-only break (Icu), the other is the break and survive to serve another day rating (Ics). If you exceed the once only rating chances are it will explode, not a good situation.

However, when you have more than once OCPD in cascade you sometimes get a higher rating depending on the break time constants and I2t limiting behaviour of them. If you take the common case in the UK of a BS88 fuse up-stream of a typical domestic DB then the fuse has this sort of a current-limiting characteristic:

View attachment 99098

If you look along the bottom X-axis for around 16kA RMS PFC and then go up to the plot for a 100A fuse you see it limits the peak current to around 10kA, then if you follow that back to the unlimited line and down you see that is equivalent to the peak of a symmetric fault of around 4.5kA RMS, so your 6kA MCB ought to survive OK.

Also remember it takes very little resistance to bring the current down, 16kA implies Zs is around 0.014 ohm, 6kA would be 0.038 ohm so 0.024 ohms more, with 2.5mm T&E at 19.5 mOhm/m for R1+R2 it is about 1.2m of cable.

That makes sense. So even with a very high fault current into the building the fault would most likely need to be in the CU or extremely close to it, with the likelihood being reduced by so much, relying on the service fuse wouldn't be too bad.