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Circuit breakers, torque and the DIN railDoes a breaker protect against earth faults?
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Our example breaker is marked B32 and BS EN 60898: a Type B, 32 A circuit breaker.
- Types B, C and D differ in how much inrush (starting) current they'll allow before tripping. Circuits need more current to get going than to keep going.
- Type B: most domestic circuits, which have very little start-up current.
- Type C: commercial and industrial, e.g. motor circuits with a high starting current that settles to a running current.
- Type D: circuits with a really high starting current.
- They all look the same apart from the letter, and fit the same board.
BS EN 60898 · circuit breakers (Types B, C, D) B up to 5 × In, C 10 × In, D 20 × In
Fitting the breaker and torque
- The breaker hooks over the DIN rail at the top and drops onto the busbar "spine". The terminal screw closes a gap that clamps onto the busbar or conductor, rather than screwing into a thread.
- Terminal screws must be tightened to the manufacturer's torque setting, in newton metres (N m). Our breaker is marked 2.3 N m on its side.
- Use a torque screwdriver set to 2.3. When you reach the setting it "breaks" (clicks and slips) so you can't overtighten. Torque settings became a big focus from the start of the 18th Edition (2018). It's part of following the manufacturer's instructions.
- Loose-looking breakers on a half-empty board aren't loose connections. They sit firm once the board is filled and the cover is on.
It sounds like "bin rail". It's the DIN rail, the standard metal mounting rail the devices clip onto.
It's often said a circuit breaker protects against short circuits, earth faults and overloads, but doesn't give brilliant earth-fault protection, especially the sensitive additional protection a 30 mA RCD gives. A CB will clear an earth fault if the fault current is high enough (low enough Zs), but it can't detect small leakage currents like an RCD can.
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