fuse discrimination

[johnmf]

can anybody tell me where to find info on fuse discrimination?

looked on some manufacturers sites but cant seem to find anything. basically want to know if a 250A bs88 will descriminate with a 400A? anybody know the answer.

its not a subject im well up on to be honest.

im also after info about how to protect a db that has a high fault current (>16KA). is it possible to place a bs88's upstream as i believe this is the case?

 

[Adam_151]

You'll be wanting to get hold of the makers datasheets for these fuses, the descrimination is then easy to check , two fuses will discriminate of the 'let through' of the smaller one is less than the 'pre-arcing' of the larger one.

Regarding back up protection, BS7671 says the let through needs to be within what the OCD in the DB its back-up protecting (and presumably any cables that might be involved in the high PSCC fault thats higher than the level the devices in the local DB can break) can stand without damage, the problem is I'm not sure these figures are easy to come by (for devices that is, for cables is obviously the old S²K² as per the adiabatic), an easier way to do it might be to find the cut-off current for the device doing the backup protection, and if its lower than the max rated PSCC of the local db, then you'd be home and dry because you have reduced the PSCC at your db (even though your testers won't really show it)


This question might be better off on the IEE forums, there are a few there like OMS and docourt that realy know their stuff where things like this are concerned

 

[johnmf]

cheers adam