The use of a separating transformer isolates the control circuits from the main supply. As a result, the Zs measurements on the secondary side of the transformer may not directly reflect the impedance back to the origin of the supply.
The high Zs readings could be attributed to the inherent impedance of the transformer and the isolated nature of the secondary circuit.
Ensure the 2A circuit breakers used on the control circuits are appropriately rated for the expected fault current levels and the impedance of the circuit.
Verify that the high Zs readings still allow for adequate fault protection. For control circuits, especially where a separating transformer is used, it might be necessary to assess the fault protection differently from typical final circuits.
BS 7671 provides guidance on the use of separating transformers and protection against electric shock.
Regulation 413.1.3.2 mentions that circuits supplied by separating transformers should provide disconnection in the event of a fault to the conductive parts.
Regulation 413.1.3.5 states that where a separating transformer supplies a control circuit, the overcurrent protection must disconnect in a time not exceeding that required for the primary circuit. Given the secondary winding is earthed at one end, standard disconnection times for TN systems may apply.
Check the distribution board and transformer manufacturer’s instructions. The design of the distribution board, including the use of the separating transformer, may have specific requirements or recommendations for achieving appropriate protection and Zs readings.
Ensure that the earthing of the secondary winding is sound and compliant with the manufacturer's recommendations and BS 7671.
Inspect all connections and wiring for integrity and proper installation practices, ensuring no loose connections that could contribute to high impedance.
Assess if the circuit design, including the transformer and protection devices, meets the fault protection requirements considering the high Zs readings.
Consider whether additional protective measures, such as residual current devices (RCDs), could provide enhanced fault protection given the high Zs values.
If the high Zs readings are found to be within acceptable limits when considering the specific nature of the installation (e.g., isolated control circuits), document the findings and rationale.
If they are not within acceptable limits, consider it a departure and document it accordingly, along with any risk assessment or mitigation measures taken.
The high Zs readings in your scenario, due to the nature of the isolating transformer, may require a detailed assessment beyond standard loop impedance expectations.
Coordination with the distribution board and transformer manufacturer, as well as adherence to BS 7671 guidelines, is essential to ensure safety and compliance. If the design inherently leads to higher Zs readings but still ensures safe operation, it might be documented as a permissible departure, backed by appropriate risk assessments and justifications.
