There is no law to say you have to upgrade any existing installation or part of it that may longer meet regulations introduced or changed since the installation. So just because you may have an old rewireable fuse board or a non-RCD protected board does not mean you have a legal obligation to change it. However it may be difficult to find an electrician who would be willing to install into an older system and sometimes they would not be allowed to without the retrofit of extra safety equipment that would be integrated into a new type consumer unit.
Saying that there are some advantages of a new consumer unit such as:
Miniature Circuit Breakers (MCBs)
Residual Current Device (RCD, RCBO, RCB)
How to check if you have an RCD installed.
An RCD is designed to protect humans from electric shock. It works by detecting a small amount of electricity leaking to earth (possibly through you) and tripping out before it can damage your body.
To check if you have one on your existing system look for switch with a button marked "PUSH TO TEST" on. This should be integrated or located near your consumer unit (fuse board) and you may have 2 or more. Simply push the button and the RCD should operate, if so, great just reset the switch that flicks down.
Click this link for The Electrical Safety First's Best Practice Guide on the requirements for upgrading a consumer unit.
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