415V switch fuse unit arc flash 1

[vijaypandiyan]

Hello electrical engineers,

In one of our 415V motor feeder, there was an arc flash. Attached are the photos. Relay in incomer saw the fault current as 12kA in all 3 phases for 100ms and fault current disappeared before the relay can trip. We see the cable from main bus connector to SFU got damaged but the cable from SFU to main contactor is OK. Fuse of R, Y phase got damaged severely. SFU is found OK when we opened it. Motor outgoing cable, motor IR tests were OK. SFU IR test is also OK.

We are confused what could have caused the accident.
My doubts are,
In SFU incoming, Y phase lug is still in SFU and connected tight but the cable got disconnected. So when it got disconnected it might have caused an arc which turned into 3 phase fault?

Requesting your valuable suggestions/opinions. Thanks in advance!

Thanks,
Vijay

Thanks,
Vijay

 

[davidbeach]

What is an SFU?

 

[ScottyUK]

'SFU' = Switch-Fuse Unit, judging from the thread title. Low-voltage switch-fuse judging from the photos.

Hypothesis:

Yellow phase lug broke away from the bottom of the switch-fuse, resulting in a single-phasing condition. Current in the healthy phases increased and the overheating resulted in failure of blue phase fuse clips, which initiated a persistent arc and caused the damage to the blue phase fuse. I can't see a thermal overload relay - is there one, and if so did it operate?

The European NH pattern fuselinks are pretty tough, it's unusual to see one with a body failure like that, especially at a relatively low fault level. I'm not familiar with L/T as a brand: how does their quality compare to the established Western brands such as ABB, Siemens, A-B, etc. for components and Ferraz / Mersen and Bussmann for fuselinks?

 

[vijaypandiyan]

Hi ScottyUK,

Thanks for the reply,
We have motor protection relay L&T make MPR300. As the power supply for this MPR is tapped from R phase and neutral and when the wires got damaged, power supply is lost to MPR and so it didnt operate.

If current in healthy phases increase, will it cause this damage? Fuse rated for 80A and 100kA and moreover, the fault existed only for 100ms. So, for such short fault, I think only arc can cause that damage.

We have lot of L&T make contactors, SFU, fuses in our refinery but this is the first time.

Thanks,
Vijay

 

[ScottyUK]

The fault only existed for 100ms at a level high enough to trigger the disturbance recorder, but is it possible that a lower-level fault was present for a period of time, and would your equipment be able to see it?

I'm not convinced that the failure of the lug on yellow phase was the initiator of the larger fault on blue phase, but I do think it may have created the conditions for the blue phase fault to develop. In my opinion the fault on blue phase fuselink and clips looks like it started as overheating damage which lead to an arcing fault between fuselink and clips: the arcing damage is very localised and hasn't spread to involve other phases or earth / neutral.

 

[vijaypandiyan]

Your first statement makes sense. Maybe the fault was present there and over heating didn't cause much current to trigger the DR.

As you say, over heating caused the blue phase fuse to damage, will the arc developed will cause 3 phase fault because the DR recorded 12kA in all 3 phases. So the single phase arc later became 3 phase arc which causes 3 phase fault, is that possible?

Thanks,
Vijay

 

[ScottyUK]

Sure it's possible that a single-phase fault evolved into a 3-phase fault. The ionised gasses from the initial fault can provide the path for the 3-phase fault to develop. It's not uncommon, although the inter-phase barriers are supposed to reduce the likelihood of it happening.

I don't see the signs of a major fault of the red or yellow phase fuse clips or the cable terminals - did the 3-phase fault arc across somewhere else, or is it just not visible in the photo?

 

[vijaypandiyan]

It should have occurred in the fuse area or power cables from bus bar to SFU, because the outgoing cables from SFU seems to be not damaged which is shown in photo. Moreover the inter barriers has gap in between them which might have helped to create 3 phase arc.

Thanks,
Vijay

 

[vijaypandiyan]

One more finding.

Found R phase cable severely burnt as shown in picture. I think, first R phase cable got disconnected which caused over heating in Y, B and caused B phase fuse to melt and cause arc?

When I look at B phase fuse, fuse top and bottom body is found ruptured and some lava kind has flown outside? what cause this type of fail? over voltage or over current?

Thanks,
Vijay