6 Common Questions about Fuse Boxes /Consumer Units

Fusebox replacement in Edinburgh

As Edinburgh electricians, we are frequently called out to replace old fuse boxes for domestic customers.  Many people find this piece of electrical equipment a bit scary. They know it protects their home but other than the Main switch (which turns off all electricity to the home) are not sure of the purpose of all the other switches. Here are answers to 6 of the most common questions we are asked about fuse boxes.

1. What is the difference between a fuse box, fuse board and consumer unit?

A fuse box or fuse board is just the outdated name (because they contained fuses) for what is now more commonly known as a consumer unit. A modern consumer unit will contain a Main switch, RCDs (residual current devices) and MCBs (miniature circuit breakers)

2. What are RCDs and MCBs and what do they do?

The purpose of RCDs (Residual Current Device) is to protect against electric shocks and electrical fires. They detect even the tiniest amount of leaking current and react very quickly to cut off all power before injury or electrocution can occur. There is usually one or two RCDs in a standard household consumer unit.

The purpose of MCBs (Miniature Circuit Breaker) is to protect electrical circuits from damage caused by too much current. They prevent overload on circuits and electrical wires overheating by cutting off the electricity as soon as a fault is detected. In newer consumer units (fuse boxes) there will be one MCB for each electrical circuit in your home.

3. What does it mean when an RCD trips?

When an RCD trips on your fuse box the RCD switch will automatically flip to the ‘OFF’ position. This usually indicates that there is a fault in one of your electric circuits. The most common reason for this is a faulty appliance although there can be other reasons.

4. What should I do when an RCD trips on my fuse box?

You can try to reset the RCD by toggling the RCD switch back to the ‘ON’ position. If the problem was a temporary one, doing this may solve the problem. If this does not work and it trips again straight away, you can

i) Try and identify the circuit with the problem

  • Flip all the MCBs that the RCD is protecting to the ‘OFF’ position
  • Flip the RCD switch back to the ‘ON’ position
  • One at a time, switch the MCBS back to the ‘On’ position.


When the RCD trips you will know which circuit has the problem and can call an electrician and explain the problem.

ii) Check for any faulty appliances

  • Unplug everything in your property,
  • Reset the RCD to ‘ON’
  • Plug in each appliance, one at a time.


If the RCD trips after plugging in and switching on a particular appliance then you have found your fault.


If you are still unable to locate the problem you should call an electrician for help.

5. How do I test the RCDs on my fuse box are working?

Above each RCD switch, there will be a button marked ‘Test’ or ‘T’. When you press this button the electricity to the circuits the RCD is protecting should turn off. The RCD button should also toggle to the ‘OFF’ position.

To switch everything back on

  • Switch all the MCBs to ‘OFF’
  • Switch the RCD back to ‘ON’
  • Switch all the MCBs back to ‘ON’


6. How often should I test the RCDs on my fuse box?

It is a good idea to test that your RCDs are functioning correctly every 3 months or so.

If you have a problem with your current fuse box or would like a quote for a new consumer unit for your property please get in touch or give Capital City Electrical Services a call on 0131 235 2608

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