I noticed that circuit breakers have a gl/gG rating. eg: gl/gG= 100A. Can someone explain the significance of this rating?
Electronic – Significance of gl/gG rating on circuit breaker
protection
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Use one designed for low voltage DC such as Eaton KD1 or this automotive type. A better option is also a Raychem Polyswitch which self resets when the current is removed. The problem with the 230V ones is the high trip current, have you really got more than 16 or 25 amps available from your 12V supply?
Edit: I didn't realise this related to an earlier question where you are talking about an automotive battery. In that case, yes, many amps are available (as opposed to a wall wart), and Andrew's comment applies - a breaker designed for AC should not be used on DC because it is a harder job to seperate the contacts on a DC current than on AC where the magnetic field falls to zero repeatedly due to the waveform.
MCBs (and fuses) have a characteristic curve showing time to trip against overcurrent. At the rated current, they should last indefinately, but as that is exceeded they trip faster. For a fast trip, you need the source to be capable of many times the rated trip current.
The thermal setting is a thermal overload protection. It has a time-delayed response. The heavier the overload, the faster it trips. It may be implemented by something like a bimetallic strip.
The magnetic setting is instantaneous short-circuit protection. It has no intentional delay. It may be implemented by something like a solenoid plunger, restrained by a spring.
Here is an illustration of a typical time-current curve for a miniature circuit breaker (Clipsal 4 series) showing the thermal and magnetic trip characteristics. Note the thermal trip takes a long time - minutes or seconds - while the magnetic trip responds in 10ms or less.
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Best Answer
This should be covered by IEC 60269. gG and gL is the same thing where gL is an older definintion.
Quote From Wikipedia article about IEC 60269
Unquote.
The gG/gL 100A that is mentioned in the question should be the fuse current rating. The current that a fuse in series with the device can continously pass through without opening.
According to the comment below, the device marked with gG/gL is a Motor Starter Protector (MSP). This should be put in series with a fuse that won't trip unless a short circuit fault occurs. Not even a stalled motor should trip it. Have a look at Rockwell Automations has a paper "193-2.10: A Guide to Understanding: Short-Circuit Protection" which deals with this. There is mentioned that the fuse should handle approx. 6 to 8 times the Full Load Current. This corresponds quite well to the MSP mentioned in the question.
Here is a picture of how the MSP and Fuse works together: