Quick answer
The shower unit has a fault — usually a failed heating element or moisture ingress. Turn it off at the isolator switch and call an electrician.
Electric showers combine high voltage electricity with water — making them particularly important to get right. If your RCD trips when you turn on the shower, stop using it immediately and switch it off at the pull-cord isolator in the bathroom. Common causes are a failing heating element inside the shower unit, moisture ingress around the shower connections, or a fault in the pull-cord switch. Do not use the shower again until an electrician has inspected and tested it — an electric shower fault represents a potential shock risk in a wet environment.
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