The present invention is concerned with a fully operational digital protective relay or Intelligent Electronic Device (IED) for protecting electrical equipment of a power distribution system. The relay comprises an input module, a processing module and an output module. Signals received from current transformers connected to the input module are evaluated by the processing module, and in reaction there to, trip signals may be output to an actuator of a circuit breaker via the output module. A base Human Machine Interface (HMI) enables a user to enter operating parameters such as delay time or nominal current to the processing module. As an option, a further HMI may be attached to the protective relay and connected, via a suitable interface for data exchange, to the processing module for the purpose of displaying protection-related information to a user. This further HMI is both optional and detachable, i.e. it may be repeatedly attached to and detached from the protection device. The detachable HMI provides for an increased flexibility in the use of the protective relay, as a user may adapt its interfacing capability by acquiring a detachable HMI of the type and at the time that suit best his evolving needs.