What you should check in your safety circuit?
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Make sure all door interlock keys are fully engaged at rest position
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Check the interlock modules in the control cabinet. Usually there are LEDs that may help you troubleshoot the circuit
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Check the interlock release keys on the interlock units are turned to the proper position. Also try removing the keys, vibration can often turn the keys prematurely.
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Check the and tighten all connections on the interlock modules and switches. Also follow the wires if possible through the interlock modules to find out which contactors to tighten up the terminals.
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Check the biggest contactors in the cabinet which power on the three phase. These usually have a lot of vibration because of the size and the wires tend to loosen up which also commonly go through the interlock circuitry.
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Next you could trace back through ladder diagram which can be tough and may not give you much information since the main part of the interlock is hard wired through the interlock modules.
Notes on some safety interlock modules
Elan type red square interlock boxes often have 4 Green LEDS on them with a fuse in the middle. The top two LED's should always be green. If A1 and A2 have power the top left will be lit. If top light is not lit then the fuse in the face of the unit is most likely blown. The other two LEDs depend on machine states.
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