russrmartin
Member
Hey guys. I have been asked by our techs to help explain this, and I am at a loss at the moment. We have heaters that are controlled by SCRs. Today they had a fire because a wire had the insulation crack and create a short to the metal clad covering. The tech investigating came to me and said the following:
When there is no load on the SCR output, he measured 230V, phase to phase. This was measured when the controlling source of the SCR was off. Meaning, that load or no load, the heaters should not be on. With the last statement still being true, he said that when a heater was connected to the output of the SCR, the voltage measurement then went to zero. Meaning, that when the heater is hooked up, and the SCR control is off, there is no voltage, as we would like. His theory was that the heater lead shorted to ground, severing the heater circuit. This opened the circuit on the SCR output. Knowing that with the SCR turned off, and no load connected, he measured 230volts, he assumes that once the heater circuit was broken, the SCR output went high. This caused a short to ground, which initiated the flame. He said he checked 4 of our units, and all 4 behaved the same. Can anyone explain to me how or why an SCR will pass voltage to it's output when the control side is not high(active, on) etc. This is beyond my knowledge, and because of the safety ramifications, this will need to be explained. Thanks.
Russ
When there is no load on the SCR output, he measured 230V, phase to phase. This was measured when the controlling source of the SCR was off. Meaning, that load or no load, the heaters should not be on. With the last statement still being true, he said that when a heater was connected to the output of the SCR, the voltage measurement then went to zero. Meaning, that when the heater is hooked up, and the SCR control is off, there is no voltage, as we would like. His theory was that the heater lead shorted to ground, severing the heater circuit. This opened the circuit on the SCR output. Knowing that with the SCR turned off, and no load connected, he measured 230volts, he assumes that once the heater circuit was broken, the SCR output went high. This caused a short to ground, which initiated the flame. He said he checked 4 of our units, and all 4 behaved the same. Can anyone explain to me how or why an SCR will pass voltage to it's output when the control side is not high(active, on) etc. This is beyond my knowledge, and because of the safety ramifications, this will need to be explained. Thanks.
Russ