Quick answer
Rainwater is getting into an outdoor light, socket, or cable connection, causing earth leakage. An electrician needs to find and seal the entry point.
When it rains, water finds its way into outdoor fittings — garden lights, exterior sockets, garage connections, or cables buried near the surface. Even a small amount of moisture in the wrong place causes leakage current that trips the RCD. Check your outdoor lights and sockets for any signs of water ingress, cracked fittings, or missing covers. Do not try to repair outdoor electrical fittings yourself — always use a qualified electrician. In the meantime, if you know which circuit is affected, switch that circuit's MCB off at the consumer unit to prevent repeated tripping.
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