Maintenance Detective: Machine Stopping
- Mac Davis
- Apr 29
- 2 min read
Suppose you have an industrial machine that's stopping with no indication of why and no fault.
Power is consistent, it just stops now and then.
What is the most probable single cause?
Answer:
It's probably a bad contact (loose wire or in a relay) in the run circuit.
The run circuit of a machine is typically a "latching circuit" that indicates to the machine that the operator is telling the machine to run when it's "high" and that the operator wants the machine to stop when it's "low."
When the machine stops with no error indication, usually it means the machine thinks it was supposed to stop and the run circuit is what told it that.
How does a run circuit work? (note the illustration)
When you push a start button (normally open), it supplies power along a single line which energizes a relay (CR1 which is a very popular convention for a start loop relay, it stands for Control Relay 1).
A contact on that relay is in parallel to the start button so it'll stay closed once energized. The circle on the diagram is CR1's coil. When power from the left rail hits the coil, the contact named CR1 closes, which powers the coil when the start button is released.
The stop button in such a circuit will be in series on this same circuit and it's normally closed. So when you push it, it opens the circuit, which releases the relay to open.
This circuit is often called the "start loop" or "run loop."
When you get no error indication, this is where you want to start. A loose wire or bad contact on the relay is a good thing to check.
Checking this circuit can be done in a power-off configuration using a multimeter. You would look for resistance across buttons or sets of buttons which would indicate a bad contact. You should also check for shorts to ground.
While every failure has a technical cause, recurring failures often point to larger issues involving process control, standardization, training, or accountability.
180 OPEX helps manufacturers reduce recurring problems, improve consistency, and build systems that reinforce critical standards across the operation.
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