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How a safety switch or r.c.d (residual current device) works!!!

Have you ever been told you need a Safety switch installed in your home or business, and wondered why? or how it works? Well in this blog I aim to explain both for you. Hopefully in a clear and simple manner.


Why is simple, it is in the name, it is for SAFETY, it is the one device that has in my opinion really moved electrical safety forwards. It protects you and your loved ones or employess.


To understand how a safety switch works firstly you need to understand the way electricity is transported throughout your home or business. Electricity is transported through wiring and it nearly always consists of 3 cores being Active (red), Neutral (black) and Earth (Green).

In a basic description:

  • Active: sends the electricity
  • Neutral: returns the electricity
  • Earth: connects to any exposed conductive surfaces, to offer a first line of protection

In a normal state of function all electricty sent by the active must return in equal volume via the neutral.

So now that we know that, how does a safety switch work?

A safety switch utilises both the active and neutral cores and heavily relies upon the earth but it is not mandatory to its funtionality. A safety switch is a monitoring device that can measure the flow of sent & returned electricity through the neutral and active cores, as explained earlier in a normal state this will alays be equal. The device that monitors this is called a differential current transformer it is found withing the workings of the safety switch.







1. differential transformer

2.differential transformer windings

3.Differential transformer core

4.A test switch (this should be activated monthly, and can be pressed by you, however if it does not activate your safety switch call your electrcian straight away)


What makes the device activate?

The device will disconnect all power to its circuit when a inbalance is detected within the differential transformer (so if the sent electricity does not equal the retunred electricity). What may cause this imbalance can be numerous things all of which can be dangerous to living creatures. I will list a few causes and try explain further.

Example 1.

You have a clothes dryer that has a metal casing, so therefore it should be earthed for safety. A active cable inside the workings of the dryer comes loose and touches the metal casing. In this state the electricity will be sent through the active but return via the earth, causing an imbalance in the transformer and activating the safety switch.


Example 2

You have your nephew over and he is very inquisitive and decideds to stick a knife into a socket outlet!!!! If a safety switch is installed it will detect that some electricity is not being returned via the neutral but rather it is going through your nephew and will automaticall shut off within .4 of a second. In this case the human recieves a electric shock but it is for a much shorter period than fuses, and in some cased a fuse wont even shut off, it will continue to send power.


Example 3

You are doing some renovations and decide to cut through a wall with your saw, but unknowingly you cut straight into a power cable. As you cut through an inbalance would be detected in the safety switch as a small amount of electricity will pass through you and once again the safety switch will disconnect the power within .4 of a second.


So looking at these example you can see why the earth is so important as it takes the electrical shock instead of a human. Compare example 1 to example 2. Please note though that Safety switches are not a gurantee there are some situation where they do not work. Caution should always be applied around electricity.




To summarise:

  1. Safety switches are great for improving the electrical safety of your house or workplace
  2. It works on detecting an inbalance in electrical flow
  3. They are not a gurantee but in the majority of cases they will save lives
  4. Always test your safety switch

I hope this has been informative, and shortly i will discuss what to do if your safety switch activates


Keith Powell


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