Seeing that Mr. Neeser has entered the fray I have a question for him or anyone else. I had a 750 va control transformer (208-120) with 2 fuseholders on primary side . One on each leg of 208volt. Sticker by fuseholder said to use Littelfuse FLQ-6 fuses. One of the fuses blew for whatever reason, Went to supply house to get some but the only had Bussman type FNQ-6 They said they were equal to FLQ. So put one in and all seemed fine. I now have 1 FLQ-6 and 1 FNQ-6 fuse in (BTW this was in Trane chiller) chiller ran about 30 hrs. and FNQ blew again. Replaced, ran another 30 hrs and blew again. Seemed like an intermitent problem on control circuit. To make short story long, replaced fuseholder, 2 trips by Trane rep ( who said to replace transformer which we did , no difference.) many blown fuses and pulled out hairs. In all this time we never blew the Littelfuse FLQ . Bussman FNQ-6 would blow nearly everytime we would power up transformer. Got more fuses, this time we got Littelfuse FLQ-6 fuses. Guess what ,fuse never blew again. Product literature suggests these two fuses are identical, Both made for high inrush current of transformers. Whats the difference? Bad batch of fuses or ?
Glenn Charter member CQA (convoluted questioners of america)
Glenn, Offhand I'd say that although obviously there is a problem, it's not fuse related. After all, the original FNQ-6 did blow and so did its FLQ-6 replacement. I assume this chiller feeder is single-phase 208v. taken off of a 208/120 wye secondary. If this were three phase, I would immediately suspect one of the phases....hang a clamp-on and look for overload, imbalance, etc.
When it comes to time-delay supplemental fuses it can be a little tricky. This is because the fuses have to meet certain performance points as detailed by UL 248. However, outside of those points, fuses from different manufacturers can vary, especially with regards to inrush characteristics.
Because of this some-manufacturers may have more time-delay for inrush considerations than others. This may even change based upon the ampere rating of the fuse and inrush point (or current value) and time duration of inrush spike. This may be the case here and is can be more of an issue for small control transformers because they can have an unusually high inrush.
However, a good comment was made by the previous person...since the FLQ opened maybe there was a problem with the transformer.
In this case, I would probably recommend using the FLQ for this application unless the FNQ can be sized a little higher.
Keep in mind, other applications with the FNQ may not have this issue, it can depend upon the control transformer used.