RCDs, MCB,s ?

What is an RCD / MCB?

All outdoor electrical distribution equipment is required by law to include some form of safety device for anyone using attached equipment. The type of device depends on where and how the distribution system is being used. Here we explain the different types of electrical safety devices as well as what is required by law and is optional.

What is an RCD?

An RCD (Residual Current Device) monitors the volume of electricity (Current/Amps) that flows in through the Live and out through the Neutral. If everything is safe and running as it should do then the Current will be the same on the way in as it is on the way out. If by accident the situation becomes critical and someone or something comes into contact with the flow of electricity, the flow of Current going through the Live and Neutral drops as some of the Current is now travelling through the person/object receiving the electric shock.

It is the RCD's job to see this change and rightly assume that electricity is leaving the circuit in a dangerous manner. The RCD then automatically trips and breaks the electrical circuit completely.

An RCD has a "time" rating. This time rating is generally 30ms (or 0.030 seconds). This is the time it takes the RCD to respond to a problem. The RCD then requires someone to manually reset it so it can continue operation. If the problem is still present then the RCD will trip again instantly. You cannot reset the RCD if the problem still exists. These devices are considered a must for almost all outdoor/environmental power applications.

RCD's are fast becoming required by law to be included in any fixed wiring application.

What is an MCB?

An MCB (Miniature Circuit Breaker) is a device that is pre-configured to allow a maximum amount of Current (Amps) to flow. To give an example, every UK plug has a fuse that acts in a similar fashion to an MCB. If you plug in a device that uses too many Amps for that circuit, the MCB will trip.

Some form of MCB is required by law whereby part or whole of the distribution unit is at risk of being exposed to a higher Current rating than it is able to handle. This is typically governed by the plug and lead coming into the unit. These components govern the overall energy that can be accessed and therefore the MCB is rated to match the lead and plug.

Overall, RCD's and MCB's compliment each other nicely. It is an RCD's job to prevent fatal accidents due to electric shock whilst it is the MCB's job to prevent damage to plug/sockets/wiring and connected equipment by being exposed to too many Amps.