Understanding switchgear jargon can be daunting, but don’t let it get you down. Here are some of the basic Circuit Breaker (CB) definitions and the characteristics of each type. This will help you better familiarize yourself with the products on the market and decide when to use them.
Miniature Circuit Breaker (MCB)
Generally rated at currents from 6A – 100A.
Generally within the fault level range of 3kA-10kA.
An MCB usually doesn’t have adjustable Trip Characteristics such as time to trip.
Comes in both Thermal or hyrdaulic-magnetic operation.
Available in single pole (SP), double pole (2P), triple pole (3P), single pole plus neutral (SP+1), triple pole plus neutral (3P + 1).
Small frame size and generally DIN mounted (Clips onto a rail).
MCCB (Moulded Case Circuit Breaker)
Rated currents between 32A – 1600 A.
Higher fault levels than MCB’s (15kA – 70kA).
Available in adjustable and fixed current rating / trip characteristics.
Comes in both thermal or hydraulic-magnetic operation.
Available in triple pole (3P) and four pole (4P).
Larger than MCB’s and fitted on a chassis to support thicker cables or busbars.
Air Circuit Breaker (ACB)
Generally rated from currents of 630A up to 10,000A.
Trip characteristics are often fully adjustable and include configurable trip thresholds and delays.
Usually include auxiliary terminals for external control and tripping.
Often contain a motor in order to toggle the breaker.
Come in draw-out or fixed mounted versions (Mounted on an angle-iron or Chassis).
Often used for main power distribution in larges installations.
Earth Leakage – Residual Current Device (RCD), Residual Current Circuit Breaker (RCCB)
Both the phase wire (line) and neutral are fed through the RCD.
The RCD trips the circuit when there is earth fault current by measuring the amount of current that flows through the phase (line) compared to the current flowing through the neutral.
Triggers within 30 milliseconds.
RCD’s are an extremely effective form of protection against shock.
Most widely used are 30 mA (milliamp) and 300 mA devices.
RCD will not protect against a socket outlet being wired with its live and neutral terminals the wrong way round.
RCD will not protect against overheating that occurs when conductors are not properly screwed into their terminals.
RCD will not protect against live-neutral shocks, as the current in the live and neutral is balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals of a light fitting), you may still get a shock.
RCDs don’t offer protection against current overloads. If a live-neutral fault occurs (a short circuit, or an overload), the RCD won’t trip, and may be damaged.
Therefore an MCB is requires in series with an RCD. A combined MCB and RCD in a single unit is called an RCCB.
For more information on circuit breakers, visit our other articles here.