How does a film cutout keep the loop alive?
In a series circuit one dead lamp can break the whole loop. A film cutout is the small one-shot device that bridges the failed lamp and closes the loop again.
A film cutout is a one-shot device wired across a lamp in a series circuit. If the lamp fails open, the cutout shorts it out, so the loop is complete again and the other lamps carry on at the set current.
1What it is
Two spring-loaded contacts are held apart by a thin insulating film and connected across the lamp. In normal running the voltage across it is only the lamp’s own voltage, which the film easily withstands, so the cutout does nothing.
2How it operates
When the lamp opens, the loop current has nowhere to go and the regulator drives the voltage across the break upwards. That voltage appears across the film. At the film’s breakdown voltage it is punctured, the contacts close, and the current takes the cutout path instead of the dead lamp. The loop is restored and the lamp stays shorted.
3Where it is used
Cutouts matter where lamps are connected directly in series, because one failed lamp would otherwise open the loop. With isolating transformers the loop stays closed through the primary on a lamp-out, so the cutout is not needed for that purpose. Some installations still fit them, so follow the project specification.
4What goes wrong
If it fails to operate, the loop stays open and the CCR’s open-circuit protection trips. If it operates early through an aged film, moisture or a surge, it shorts out a good lamp. When it does operate, the shorted lamp is dark and easy to miss until it is inspected. The brief open at the moment of breakdown can also trip protection. A cutout cannot be reset, so it is replaced with the lamp.
| Stage | Lamp | Film cutout | Loop |
|---|---|---|---|
| Normal | Lit | Film insulates, contacts apart | Closed |
| Lamp fails | Open | Voltage builds across the film | Open |
| Breakdown | Open | Film punctured, contacts close | Restored through the cutout |
| After | Out until replaced | Spent; replace with the lamp | Closed, circuit load lower |
Frequently Asked Questions
Q1What is a film cutout?
A small one-shot device wired across a lamp in a series circuit. Two spring-loaded contacts are held apart by a thin insulating film. If the lamp fails open, the film breaks down, the contacts close and the lamp is shorted out so the loop is complete again.
Q2How does it operate?
When the lamp opens, the loop current has nowhere to go and the regulator drives the voltage across the break upwards. That voltage appears across the film. When it reaches the film's breakdown voltage the film is punctured, the contacts close, and the current takes the new path through the cutout instead of the dead lamp.
Q3Why do some series circuits need one and others not?
Where lamps are connected directly in series, one failed lamp would open the whole loop, so a cutout bridges it. With isolating transformers the loop stays closed through the primary when a lamp fails, so the cutout is not needed for that purpose. Some installations fit them for other reasons, so follow the project specification.
Q4What happens if a film cutout fails to operate?
The loop stays open, every lamp on the circuit goes out, and the regulator's open-circuit protection trips. The circuit is then isolated and the break is found by testing each section.
Q5Can a film cutout operate when the lamp is still good?
Yes. An aged or damaged film, moisture, contamination or a voltage surge can break it down early, which shorts out a working lamp. A lamp that is dark with a good bulb is a reason to check the cutout.
Q6Can a cutout be reset?
No. It is a one-shot device. Once the film has broken down the contacts stay closed, so the cutout is replaced along with the lamp. Film rating and replacement practice come from the manufacturer, and the circuit is isolated and proved dead before any work.
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