Quote from ERMCO Instruction Manual #105:

Some breakers are provided with a red signal light which gives warning that the load has reached a value near the tripping point of the breaker. The signal light remains lighted until reset (turned off) by means of the breaker operating handle. Transformers should not be operated under load conditions that will cause the red light to appear frequently, since it indicates an overload on the transformer. When such a condition exists, it is recommended that a larger transformer be substituted to avoid impairing the life of the smaller unit.

To reset the signal light without disconnecting the load, rotate the handle to "L" (light), and then return to "C" (closed). If the light fails to go out, the transformer is still overheated. To check the signal light bulb when the transformer is in service, rotate the handle to "L" (light). The light should turn on. If it does not, the bulb should be replaced.

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