'Won't start' is usually one open link in the string

A machine that 'won't start' is usually one open link in the control-power string — a blown control fuse, an E-stop in the chain, or a single missing permissive. Read the string on the print and the open link stands out.

A faultline101 training module · the skill behind six hands-on scenarios

A machine that 'won't start' is usually one open link in the control string — a blown control fuse, an E-stop in the chain, or a single missing permissive. Read the string and the open link stands out.

The idea

Control power starts at a control transformer (CPT) that steps the line down to a safe control voltage, protected by its own fuse and referenced to a control ground. If that transformer or its fuse is gone, nothing in the control circuit works — no permissive can even be evaluated.

Downstream, the start of most machines depends on a permissive string: E-stops, guard interlocks, safety-relay contacts, and process permissives wired in series. Every one must be satisfied for the coil at the end to pick up. A single open link — one E-stop, one guard, one permissive — reads as 'the machine won't start,' with no obvious symptom until you trace the string.

The print lays the whole string out in order. Read it from the control transformer through the permissives to the coil, and the open link is the one where your expected control voltage disappears. And the rule from voltage-tracing still holds: you meter the string, you never bypass an E-stop or interlock to force a start.

Six faults you'll work in this module

You don't learn this by reading it — you learn it by doing it on six different faults, each drilling the same skill until it's instinct:

1. One open link in the string. The string trace named the open permissive.
2. No control voltage at all. No control voltage traces back to the CPT fuse.
3. The E-stop string. Metered safely to the exact open station — never bypassed.
4. A control ground fault. Reading the grounding on the print explained the sneak circuit.
5. Present, but still won't start. Reading the contact's operating state found the real block.
6. Cold: won't start, find the link. Cold machine, control string traced safely to the open link.

The mistakes it kills

• Chasing the load when there's no control voltage at all because the CPT fuse is blown
• Overlooking one open permissive or interlock in a long series string
• Bypassing an E-stop or guard interlock to force a start instead of finding the open link

Why it matters: Most 'won't start' calls are a single open link in the control string — reading control power and the permissives on the print finds it fast, without ever defeating a safety device.

More: Ladder vs wiring vs one-line · Get the comic · All troubleshooting guides.