SWA supply cables between buildings will not disconnect in required time

[Vortigern]

Doing calcs on swa supply to building as sub main from main three phase squard D board for single phase supply off of TN-S supply. I used cable mate from jarsoft and looked up various tables for disconnection times. In all case scenarios it tells me it will not disconnect within the required time. The Ipf at .211K is insufficient fault level. This is based on calculating EFLI on 16mm² being 1.091 and the max Zs Ohms being (table 41.3) 0.69. Vd has been calc. and is ok. This also assumes 5s disconnection time based on being a distribution circuit. It is also based on using type B 61009. Which is what is throwing me as at 1667 ohms it should be absolutely fine. Am I missing something here or is the Cable Mate software wrong?
See calc sheet included.

 

[westward10]

Good to see you back mate.

 

[Strima]

Can you drop the design current down a few amps?

 

[Vortigern]

Can you drop the design current down a few amps?

I could drop it down to 50A but still the same problem. Given the max load technically 63A is the optimum design current. I have even tried it at 20m run again the same problem. I am a bit flummoxed. I really must be able to supply a sub-main with SWA I do it all the time with no problem. What am I missing.

 

[Baddegg]

Welcome back @Vortigern

 

[Vortigern]

I could drop it down to 50A but still the same problem. Given the max load technically 63A is the optimum design current. I have even tried it at 20m run again the same problem. I am a bit flummoxed. I really must be able to supply a sub-main with SWA I do it all the time with no problem. What am I missing.

Edit: actually that is wrong, it does comply at 50a

 

[MFS Electrical]

In real life it will be fine as it’s an RCBO. However Technically it’s a bad design because it doesn’t satisfy the max Zs as prescribed for a type B 63amp “MCB” and another method of protection may satisfy the calcs for max disconnection time such as a different rating or even a fuse. But because you are using the RCBO the maximum Zs would be 1667ohms And effectively it cancels out the fact that it doesn’t comply with the max Zs for a 5second disconnection time. Reason being the max Zs tables do not take RCDs into account and is solely focused on the fault current that would effectively trip the MCB part of the RCBO.
Only thing I’d be concerned about is that the nuetral prospective short circuit current was high enough to operate the device in short circuit conditions but as the line-nuetral prospective fault current is usually much higher than the Ipf you should be grand. of course once installed I’d measure it just to be sure but I don’t think I’ve ever seen a condition where a dead short wouldn’t operate an MCB. (Unless it was a Dorman smith loadmaster,loadline or loadlimiter but that’s a story for another day)


Little off course but provides a little background;
When carrying out an EICR in such a circuit I’d code the max Zs as a C3 because the RCBO kicks that into touch. If it weren’t for the RCBO it would be a C2. Obvious fix to the C2 would be swap the MCB for an RCBO

 

[Vortigern]

In real life it will be fine as it’s an RCBO. However Technically it’s a bad design because it doesn’t satisfy the max Zs as prescribed for a type B 63amp “MCB” and another method of protection may satisfy the calcs for max disconnection time such as a different rating or even a fuse. But because you are using the RCBO the maximum Zs would be 1667ohms And effectively it cancels out the fact that it doesn’t comply with the max Zs for a 5second disconnection time. Reason being the max Zs tables do not take RCDs into account and is solely focused on the fault current that would effectively trip the MCB part of the RCBO.
Only thing I’d be concerned about is that the nuetral prospective short circuit current was high enough to operate the device in short circuit conditions but as the line-nuetral prospective fault current is usually much higher than the Ipf you should be grand. of course once installed I’d measure it just to be sure but I don’t think I’ve ever seen a condition where a dead short wouldn’t operate an MCB. (Unless it was a Dorman smith loadmaster,loadline or loadlimiter but that’s a story for another day)


Little off course but provides a little background;
When carrying out an EICR in such a circuit I’d code the max Zs as a C3 because the RCBO kicks that into touch. If it weren’t for the RCBO it would be a C2. Obvious fix to the C2 would be swap the MCB for an RCBO

I do agree with your contribution and did have a little chuckle about it should be ok on the L-N fault and measure it when installed and had this horrible feeling if it did not measure compliantly when installed then it's a oh *eck moment. As the OP said at 1667 ohms no problem. Again agreed not the best practice though. But again a dead short will see off most MCBs or certainly should do.

 

[MFS Electrical]

I do agree with your contribution and did have a little chuckle about it should be ok on the L-N fault and measure it when installed and had this horrible feeling if it did not measure compliantly when installed then it's a oh *eck moment. As the OP said at 1667 ohms no problem. Again agreed not the best practice though. But again a dead short will see off most MCBs or certainly should do.

Don’t want you citing me as the reason if it all went to s**t you see. never assume what I probably should have said was measure the PSCC at source and calculate the PSCC at the end then go from there and double check once installed and by the way I was already typing before you came back with the 50A so win win bang it on a 50 and everything’s all good.

 

[Dan]

Doing calcs on swa supply to building as sub main from main three phase squard D board for single phase supply off of TN-S supply. I used cable mate from jarsoft and looked up various tables for disconnection times. In all case scenarios it tells me it will not disconnect within the required time. The Ipf at .211K is insufficient fault level. This is based on calculating EFLI on 16mm² being 1.091 and the max Zs Ohms being (table 41.3) 0.69. Vd has been calc. and is ok. This also assumes 5s disconnection time based on being a distribution circuit. It is also based on using type B 61009. Which is what is throwing me as at 1667 ohms it should be absolutely fine. Am I missing something here or is the Cable Mate software wrong?
See calc sheet included.

Welcome back to the forum matey. Nice to see you managed to get back on.