I am hoping that Neil Estrick might comment about my recent experience with a Skutt KM-1 wall mounted controller, as well as anyone else who may have an opinion about this incident, and if I should be concerned about using the controller for my larger kiln.
I bought a new Skutt KM-1 wall-mounted controller in order to automate firing of a couple of electric kilns I have. One is a medium size older Knight, maybe 4-5 cubic feet, and the other is an Olympic 25 Oval, over 12 cubic feet. The power draw is stated to be 48 amp, and the recommended breaker size for the Olympic is 60 amps. The Knight is far less and does not play into my concerns.
I had successfully fired the oval several times to Cone 6 (rated cone 8 kiln) using the KM-1. Then on my last glaze firing the controller failed. I went to check the kiln and the KM-1 display was dead, rather than flashing temperature and firing time. There was a smoky electrical odor in the air. I checked to see if a breaker had tripped, and it had not. I checked to see if the control fuse had blown, and it had not. While replacing the fuse, I noted the display came back as I pushed to twist it and lock it in. When I released it, the display went black again. I found that on if I pushed and held the fuse that I could power the display, so I did so and checked for error codes. It showed a power failure, but we had no power failure during the firing. Mystified, I tripped the breaker and resumed my evening activities. The next morning investigating inside the controller, I found that the wiring leading to the contactor inside the controller had melted insulation, so without doing any more I packed it up and off to my supplier for warranty service. I probably should have taken a picture, but didn't think of it at the time. The contactor and a pilot relay and the wiring were all pretty smoky looking, and I was concerned I might void my warranty so I closed it up quickly.
From what I have been able to find out, the contactor and the pilot relay were replaced, and the wiring was cleaned up, shortened a bit and reconnected. Apparently the melting had disconnected the control circuit wiring, so the pushing had powered it back up until you let go. I have two firings since reinstalling the controller, both on the smaller Knight kiln, one a test fire and one a bisque firing. No problems with either firing. I have yet to fire the larger oval, and am a bit concerned about it.
While investigating and waiting for the controller to come back to me, I found the model of contactor (Square D 8910DP42V09) is rated at 50 amps per the Skutt marketing data, but in checking the ratings from the manufacturer, the contactor is rated at 40 amps inductive and 50 amps resistive. Skutt knows that it is used on the Olympic and I sent a electrical spec sheet. Since my 48 amps is right at the 50 amp resistive rating, should I be concerned?
Given the failure, and the melted wiring, I don't want to be creating a safety issue while firing. I can see where if the contactor failed closed, that this would melt the wires, but it never tripped the circuit breaker. Apparently the control circuit wiring burning had dropped out the power. So, fluke one time incident, or potentially repeating problem.
Sandy Springs, Georgia, USA