Beginners Guide to Circuit Protection

A quick and easy guide to isolators, fuses, circuit breakers and residual current devices in ac applications for fault protection.

Circuit protection

Electricity is dangerous stuff, at AC voltages of 230V (the standard across Europe) 50mA will stop the human heart of a healthy adult. All pieces of electrical equipment therefore have to meet rigorous safety standards to be sold in Europe and carry the CE mark. An essential part of the 'inherent' safety of a device is some form of protection under fault conditions.

There are many types of fault and many different types of protection this page will deal SIMPLY with the principles involved.

For current to flow a complete circuit must be created starting and finishing at a source of power. Any break in the chain will stop all current flowing in the chain.

The simplest form of circuit protection is therefore a switch

Switch, Isolator, Breaker

A simple mechanical device which opens an air gap in the circuit and is manually operated. It is important to note the size of the air gap is VOLTAGE dependant, higher voltages can arc across small gaps. The size of the switch terminals are CURRENT dependant, terminals too small could weld together on over current. The nature of an isolator is normally throw or spring assisted, in other words it has two steady states open and closed and cannot be left half way between. The size of the 'spring' and the lever needed to activate the switch are also CURRENT dependant, a large current device needs to have a sufficient 'breaking capacity' to perform a clean switching operation.

Since a switch is a manual device it is used normally only for an elective purpose, however many switches have the ability to be locked out, in other words a key is required to turn them on. This locking out procedure is used by electricians working on a piece of equipment, they first turn off the switch and then padlock it to prevent accidental switch on while they are working on the equipment.

Magnetic Circuit Breaker (MCB), also Miniature Circuit Breaker

The simplest and most common form of 'automatic' switch is a Magnetic Circuit Breaker (MCB). In addition to the manual switch a magnetic circuit breaker contains a small electromagnet coil which moves its magnetic core to automatically disconnect on overcurrent. Purely magnetic circuit breakers are resilient to small overloads and surges of current but react very quickly (a few milliseconds) in fault scenarios, as such they are the work horse of the safe electrical circuit.

Circuit breakers come in 3 main 'types' or 'curves' which defines there speed of action at overcurrents.

Type / Curve Short Circuit response Applications
B / 2 3-5 times current rating Circuits with moderate inrush, mixed lighting heaters and other resistive loads
C / 3 5-10 times current rating Higher inrush mildly inductive loads, motor drives, ballast lamps or large amounts of switching
D / 4 10-20 times current rating Abnormally high inrush due to highly inductive loads, transformers, motors and non power factor corrected thyristor drives

Thermal-Magnetic Circuit Breaker (MCB)

A Thermal Magnetic circuit breaker has two automatic disconnect devices, as well as the coil described above it also contains a bimetallic strip which the current passes through. The two metals are chosen so that as they heat up in response to the current flowing, as the temperature rises the strip curls, if a large enough current flows disconnection occurs. This thermal type of circuit is predictable and has the added benefit of detecting even quite small overloads. The curvature of the strip is slow, so that overloads of 150% which last only a brief period will not disconnect, long term overloads (e.g. 125% for several minutes) will. The thermal disconnect also has an added benefit in that it effectively automatically derates devices in high ambient temperatures.

Thermal Circuit Breaker

Purely Thermal circuit breakers are used in low cost low current applications, they work with just the bimetallic strip and as such are NOT suitable for circuits where short circuit currents could be large enough to weld together the contacts.

Residual Current Device (RCD), Residual Current Circuit Breaker (RCCB), Ground Fault Circuit Interrupter (GFCI)

In earthed electrical systems, (UK mains and many other countries) it is possible to fit a further form of protection, since any current flowing 'out' of the live should be definition exactly match the 'return' on the neutral, any imbalance between these two values constitutes a fault. A Residual Current Device (RCD) uses an electronic circuit to detect even the smallest imbalance between the live and neutral conductors and if it reaches a trigger level disconnects the circuit. Again this disconnect is in the order of milliseconds and RCD's can be specified to sense fault levels as low as 5mA ( typically 30mA ). In modern electrical circuits many devices contain filtering circuits for EMC compliance, some of these circuits contain deliberate 'Earth Leakage' leading to nuisance trips of RCD's.

A great deal of confusion is caused by the fact that both Magnetic and Thermal-Magnetic circuit breakers are called MCB's, in addition it is possible to get a combined MCB and RCCB in one device (Residual Current Breaker with Overload RCBO), the principals are the same, but more styles of disconnection are fitted into one package.