The clever chaps over at the Electrical Safety Council have provided a simple checker you can use to ensure you are not overloading your extension cables.
If you are using a reel type extention that the rating for it wound up will be significantly less that for it fully unwound, also if you have a very long cable that would also have a lower rating unless thicker cable has been used.
Don't use extensions if plugs, sockets or cabling is in anyway damaged.
More extension cable safety advice.
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