Dimmers burning...

[headcrab]

For background:
Our stage lights are dimmed by three NRD8000 packs. Recently, on two channels, the internal fuses blew but not the outside breaker. I know the dimmers have not been overloaded, there are only two 575W lamps on each channel.
When I went to replace the fuse, I noticed that the solder anchoring the fuse holder in place had been flowing out. With 10 amps flowing, that discolored the board.
My first question is why do the fuses blow at 10 amps when they are rated for 15?
Has anyone else experienced this failure mode and what did you do?

 

[Van]

This is not the first time this particular failure has been reported on this board. Run a search I seem to remember someone having the same issue about 2 years ago.
I've had similar issues with other NSI dimming poducts so I'm not too surprised to find you having issues with the NRD8000

 

[epimetheus]

We had NSI NRD packs at my previous church, both 8000's and 9000's. They weren't all that great. By the time I left the church, they had developed a nasty flicker at anything less than 100%. One of the 8000 packs also had overheating issues even though it was definitely not overloaded. 8000's are only rated at 1200W per channel, so (2) 575's are getting close to the channel limit of an aging dimmer pack.

 

[David Ashton]

A fuse blows by heat, so if there is a bad joint or contact on the fuse this will cause the fuse to heat up, effectively de-rating the fuse.

 

[gordonmcleod]

Often a fuse holder especially the small ABC style will develop corrosion or pitting on the contact surfaces and that will cause resitance and there fore heating causing fuses to melt and blow

 

[tyler.martin]

I have 3 NRD8000's too, and one of them has completly kicked the bucket...there was an lightning storm here, and the blown fuse is now welded to the holder among other problems... I am now waiting on my new strand A21 system!

 

[Slitterst]

Well, it looks like the product in question has a history of doing this.

That said, just because you put two 575 lamps on the dimmer doesn't mean that there was not an overload or short circuit that caused the problem. Regardless of the litany of problems others have mentioned with those dimmers, you need to check your cabling and instruments to make sure there was not a fault in the equipment that caused those fuses to blow.