Power Distribution Circuit Breaker Protection - RCDs, MCBs, and RCBOs

Our range of PCE MERZ, ISCHL, IMST, and ST ANTON Power Distribution Units feature 3 different types of circuit protection: RCDs, MCBs, and RCBOs.

Here we'll give a quick overview of the differences between these devices and why they're important to understand and use.

Should you have any questions or require advice please get in touch.

RCD (Residual-Current Device)

These are safety devices designed to break an electrical circuit very quickly when they detect there is current leakage to ground. They do this by continually monitoring the current of the live supply and neutral return of a circuit. If the current becomes unbalanced, which is indicitive of the current leaking to ground, then the RCD trips and breaks the circuit. The time it takes for these devices to detect a leakage and break the circuit is so fast that it's measured in milliseconds.

First and foremost this is to protect against ongoing electric shock to users which could prove fatal, but it also protects the electrical appliances connected to the circuit. Should the RCD trip, it can be reset once the fault is cleared. RCDs also feature a test button, allowing you regularly check the device is working properly and the circuit is safe to use.

All final user circuits rated 32A or lower should be protected by an RCD.

 

MCB (Miniature Circuit Breaker)

An MCB is designed to trip and break a circuit in the event of an overload or electrical short fault. All the MCBs used in our range PCE distribution boxes are thermal-magnetic, and have a current rating and curve type.

The magnetic part of the MCB is designed to break the circuit instantaneously in the event of a large current surge which is typically caused by a short circuit. Whereas the thermal part features a time response curve, meaning that it will trip faster for higher over-currents, and slightly slower for lesser but more sustained over-currents. The purpose of this is to allow for short spikes in current or in-rush often found when switching on motors or non-resistive loads.

Only two types (curves) of MCBs are used in our distros, either B or C. Type B is designed to instantaneously cut off when the current exceeds 3-5 times its rating, type C will trip when it exceeds 5-10 times its rating. Any over-current that exceeds an MCBs rating will trip it, but this is where the thermal curve comes in to play and is best shown by the above graph.

You can ascertain an MCB type by checking the information on the front labelling. There should be a note of the curve type, followed by the rating in amps. For example, C32 would denote a type C MCB with a rating of 32A.

If an MCB trips then it can be reset once the fault has been cleared.

 

RCBO (Residual-current circuit breaker with over-current protection)

Simply put, an RCBO combines the features of both an RCD and an MCB into one device, and can detect either current leakage or current overloads.

There are different types of RCBOs, but you will only find type A RCBOs in our PCE Power Distribution boxes which are sensitive to AC and pulsating DC faults.

RCBOs also have a curve type rating for their MCB component response times. This is explained in the above section on MCBs and is denoted in the same way on the front of the RCBO label.

Distros fitted with RCBOs for all circuit outputs tend to be the most expensive, but the benefit of having them is that if one of the outputs were to develop a current leakage fault, then only this output would trip because it has its own dedicated RCD as part of the RCBO.

The rest of the distribution unit outputs would remain unaffected and continue operating normally, which is hugely advantageous for events use and other environments where seemless operation is paramount.

Plus, this feature aids in finding the location of the fault more rapidly as you will have already identified which output the faulty appliance is connected to as opposed to the trial and error process of checking each output if the distro utilises an overall RCD.