Unusual fuse selection? 1

[MetalworkerMike]

I've got two Polytruder extruders and they both have a bunch of Dayton blowers on them (small 110V 7A FLA units) for cooling the barrel. I've found that each of these blowers is running through a Cooper GMA 10A fuse on the hot line. These are wee little 5mmx20mm fuses so they could fit them into a fused terminal strip (I hate those!), and more importantly they are fast-acting fuses. This seems odd to me. I'm planning on replacing them with GMC 10A fuses (medium delay) for a wee bit of extra inrush protection, unless someone here can think of a reason not to, but can anyone think of any reason why these GMA 10A fuses might have been necessary in the first place? Or is it more likely that they just happened to be easier to source (i.e. on the shelf at the time)? I've got several blown fuses to replace at the moment.

Mike

 

[HCBFlash]

sounds like a lousy choice by whomever put those fuses and blocks in there. I'd put in a class CC fuse for each and -Working strictly out of my head here- use an FNM equivalent, and size it pretty close.

 

[rawelk]

If there is/can be room for them I second what HCBFlash says, install different holders, and go with a 1-1/2" x 13/32" fuses. If there isn't enough room, then the GMC10 you've indicated will probably be a workable compromise.

I've never seen 5x20mm fuses in motor circuits (nor do I want to

With fan-based extruder barrel cooling keep in mind not to set PID temperature controller cooling cycle time below 20 seconds or so. A 20 second cycle time is 3 starts/minute when in cooling mode, and not "saturated" (that is, not calling for 100% cooling), or 180 starts/hour.

A fuse element grows and shrinks a little bit every time load is applied, current flows through it, and it gets hotter (then cooler, when current is turned off). Think of the effect as a greatly attenuated version of what happens when turning on and off an incandescent light bulb. It 'work hardens' the metal, and especially when operated in lower ambient temperatures this leads to a higher fuse mortality rate.

Motors are worse in this respect than resistive heating elements due to inrush current, but sometimes you can run into problems on the flip-side ... one case in point.

A couple of years back we installed a extruder control system replacement using 10x38mm semiconductor fusing for barrel heat zone protection, solid-state power blocks, and used an existing clean air duct system (untempered outside air) for cabinet cooling. Nothing we haven't done before, but as winter approached heater zone fuses started popping like, well, popcorn.

No identifiable grounding problems were found, fuse amp ratings were well suited to both load and power controllers, and fuse failures were happening on a number of different zones. I happened to be troubleshooting an unrelated problem one day, and noticed the system integrator had used a heating cycle time of 20 seconds.

Changing heat cycle time to 5 seconds cured the fuse blowing problem. Wild, eh?

 

[MetalworkerMike]

I don't think I have room for CCs, but I'll double check today. I thought it seemed ridiculous to use those fuses, but I always have this nagging thought that _surely_ the guy who designed the cabinet knew something that I don't... Anyway, for now the GMCs will give me a bit of leeway, and I'll double-check the cycle time on the controllers. They _do_ seem to be lacking in hysteresis, to me, and it's been in the back of my head to take a look at that. Haven't done it yet, though.
Rawelk: Maybe the short cycle time on your heaters were within the 'slow-blow' period so they were acting like higher amperage fuses? This assumes they were slow-blow fuses or at least act like them to a degree...

Mike

 

[rawelk]

The fuses in question were I2t semiconductor fuses, and their characteristics are both fast clearing and (extremely) current limiting. They are used to protect solid-state power devices, in this case, 3 pole, 55A, back-to-back SCR power modules connected to three phase resistive heaters (extruder cast-in barrel heaters), and single phase adapter and die zones.

Solid-state relays and contactors (SSRs, and SSCs) don't take kindly to heat, and short circuited loads dissipate massive thermal energy inside them until the fuse clears.

I2t fuses are selected so their total clearing amps squared per second rating (typically specified at 8.3 ms, 1/2 of an AC cycle at 60 Hz) is less than the SSR or SSCs specified I2t withstand rating.

For instance, if a SSR power block has an I2t rating of 2000, then the fuse's I2t rating must be lower than this for the power block to have any chance of survival.

 

[dw51]

From a compliance standpoint, it sounds like your equipment would fall under UL508A. 508A does not accept miscellaneous class fuses for motor protection.

If I read your fuse spec correctly, it has an interrupting rating of 200 Amps. Although the fuse is listed by UL, it is not listed for this application. Your assumption that 'the guy who designed the cabinet knew something' may be a bad one. I'd look to install fuses that not only have the operating characteristic you want but also provide adequate short circuit protection.

 

[MetalworkerMike]

The interrupt rating of these things is only 200A? I hadn't checked that. That is nowhere close to decent. I'll have to figure out a way to fit some CC-class fuses in there. Somehow.

Mike