BS88-2 20A NITD Vs BS1362 13A

[late]

Hi All,
can anyone help?

So at work on the shop floor there are about a dozen weld bays all fed via a Busbar system, each bay has 1 double gang single phase 230v socket protected via NITD 20A fuses to BS88 (each socket is independently fused), the single phase sockets are generally used for the grinders various brands and occasionally a radio, the plug fuses in the grinders are 13A BS1362 as specified in the manuals for the grinders.

The NITD 20A BS88 fuses in the busbar outlet to the sockets blow on a regular basis (about 1 every 2 - 3 days) there is no pattern to this it can be any of the 12 bays.

They only blow when using the grinders, we have identified an issue of metal grinding dust contaminating the grinder and have implemented a weekly clean of the grinders inside.

I guess my question is why is it that it is always the 20A bs88 fuse always blows and never the 13a bs1362?

even though the manufactures of the grinders specify 13a fuses, we have tried down rating the plug fuse to 10a bs1362 and even then the 20a still goes first. It is far more effort in replacing the fuse on the busbar outlet than the plug fuse but in anycase need to eliminate it.

I have looked at the curves for bs88 fuses in BS7671 but cannot see any for the BS1362 to compare. any ideas???

thanks in advance for taking the time to read

Regards
Late

 

[Pete999]

time curve bs1362 fuse - Google Search - https://www.google.co.uk/search?q=time+curve+bs1362+fuse&client=firefox-b&tbm=isch&imgil=uM2LYUl1hoT9XM%253A%253BqF9c8CXlToaezM%253Bhttp%25253A%25252F%25252Fwww.aboutelectricity.co.uk%25252Farticles.php%25253Farticle_id%2525253D43&source=iu&pf=m&fir=uM2LYUl1hoT9XM%253A%252CqF9c8CXlToaezM%252C_&usg=__aUXJfgI8lSs4CznAxdWWYhgTv2k%3D&biw=1708&bih=794&ved=0ahUKEwiluLfv4_7VAhXhC8AKHRFeCY4QyjcITA&ei=jJemWaXCB-GXgAaRvKXwCA#imgrc=uM2LYUl1hoT9XM:

 

[telectrix]

what size cable feeds these sockets? and is it radial or ring?

 

[Richard Burns]

I have tried an overlay of BS88-2 (black) with BS1362 (purple) and it seems to show discrimination (just) until you get to short circuit levels of fault current.
Therefore the 13A should go first, however at full fault current any melting fuse is going to to be close to going and it will be close which goes first.
Perhaps using motor rated 20A fuses, if this is acceptable for the circuit characteristics, may be an answer.

I think preventing the short circuit in the first place may be better, perhaps finer filters over the grinders motor intakes (with out restricting air flow).

 

[late]

what size cable feeds these sockets? and is it radial or ring?

Hi thanks for your reply, the sockets are just 1 per bay so they were wired as a radial using 4.0mm, the length of each circuit is very short max 4 mtrs. There are 2 busbars (6 bays ber busbar) both fed via 35mm 5 core SWA which was some cable we reused from a decommissioned installation and fused at 100a per bussbar.

 

[late]

I have tried an overlay of BS88-2 (black) with BS1362 (purple) and it seems to show discrimination (just) until you get to short circuit levels of fault current.
Therefore the 13A should go first, however at full fault current any melting fuse is going to to be close to going and it will be close which goes first.
Perhaps using motor rated 20A fuses, if this is acceptable for the circuit characteristics, may be an answer.
View attachment 37980
I think preventing the short circuit in the first place may be better, perhaps finer filters over the grinders motor intakes (with out restricting air flow).

Hi thanks for your help, we did briefly try motor a rated fuse in one bay (20/32) but this did not help, which originally led us to the thinking that it was short circuit as opposed to starting current of the grinder.

As you say stopping any potential short in the first instance is clearly the way but im not sure of filtering the grinders would be acceptable due to air flow restriction, things have improved a bit with weekly cleaning of the grinder internal connections etc. Maybe we will have to turn to twice or even 3 times cleaning.

as i mentioned in my original post that we did also try 10a 1362 in the plug but still no joy, maybe we might get away with 7 amp plug fuses but i suspect this will be pushing it.

thanks again for your help, much appreciated.

 

[davesparks]

Are both sockets connected to the same 20A fuse and are they both in use at the same time? The combined load could be taking it over 20A.

Could this be a bad batch of 20A fuses?

Are the fuseholders in good condition? Could extra heat buildup in the fuse holder from a poor connection be responsible (this is a pure guess on my part)

If it is metal dust building up in the grinders then you may well need to blow them out daily or even more regularly than that.

 

[late]

Are both sockets connected to the same 20A fuse and are they both in use at the same time? The combined load could be taking it over 20A.

Could this be a bad batch of 20A fuses?

Are the fuseholders in good condition? Could extra heat buildup in the fuse holder from a poor connection be responsible (this is a pure guess on my part)

If it is metal dust building up in the grinders then you may well need to blow them out daily or even more regularly than that.

Hi reading back my original post it does sound unclear..... each weld bay has 1 x double gang RCD socket wired as a radial circuit on its own dedicated 20a NITD bs88 fuse, so only one appliance in use at any one time on each radial socket/circuit. the fuse holders in the busbar plug in units are in good condition and are screw terminal, and there is no evidence of heat build up. you are most likely correct on the grinding dust, some that i have inspected, have clear tracking through the metal dust around where the cable terminates inside the grinder. maybe we should look at coating the electrical terminations in some form of insulating resin?
thanks for your reply much appreciated.

 

[davesparks]

If tracking across the insulation has already occurred then it will have left a conductive path which will track again and again. It will take some serious effort to clean it off so that it doesn't track again.

It may be possible to apply a conformal coating such as is used on pcbs to protect them, but it will only work if the area it is applied to is completely clean, otherwise it will just continue to track along the same path under the coating.

 

[DPG]

May be worth consulting the manf. of the grinders. They may have come across this before?

 

[davesparks]

This problem has been mentioned on here before, I've got a feeling it was by someone who is no longer a member. I think part of the solution was blowing the grinders out with an airline regularly, possibly as regular as every couple of hours of use.

 

[Lucien Nunes]

If conductive dirt or tracking in the grinders is blowing fuses on a regular basis they are faulty and should not be used. This would not be a normal occurrence - any reputable power tool should be able to do its intended job for a reasonable period without blowing its fuse. You might find that replacing tracked parts instead of cleaning them, results in a long period of fault-free operation. I assume we are talking about handheld grinders with universal motors, not bench grinders? The latter usually have totally-enclosed induction motors and should not experience conductive dirt buildup at all.

If the work produces so much ultrafine conductive dust that it is impossible to keep electric tools free of insulation faults, perhaps air tools would be superior? And/or integral extraction?

 

[DPG]

Air tools does sound a better idea.

 

[late]

Thanks everyone for your help, honestly im at a loss, im sure that there is so much conductive dust that maybe air grinders are the answer but this is not my call within the company. the grinders are not faulty, it happens with new grinders etc.

 

[late]

If conductive dirt or tracking in the grinders is blowing fuses on a regular basis they are faulty and should not be used. This would not be a normal occurrence - any reputable power tool should be able to do its intended job for a reasonable period without blowing its fuse. You might find that replacing tracked parts instead of cleaning them, results in a long period of fault-free operation. I assume we are talking about handheld grinders with universal motors, not bench grinders? The latter usually have totally-enclosed induction motors and should not experience conductive dirt buildup at all.

If the work produces so much ultrafine conductive dust that it is impossible to keep electric tools free of insulation faults, perhaps air tools would be superior? And/or integral extraction?

sorry yes hand held grinders generally around 1000w