RCD Blinding

Blinding occurs when an RCD cannot see the fault current. It can happen for a host of reasons.

Electric vehicle (EV) charging is probably the best known cause of blinding.

EV charging equipment relies on automatic supply disconnection to protect the user and other members of the public in the event of certain faults. To provide shock protection, RCDs must operate before the residual current reaches a dangerous level and within the defined disconnection time. This protection feature will be ineffective where inappropriate RCDs are specified or installed.

At Western Automation we work with our customers to determine the best RCD technology for the specific application. The level and characteristics of the currents in normal operation and under certain fault conditions can result in reduced sensitivity or blinding due to magnetic saturation of the RCD sensing circuit and therefore reduced protection for users.

The incidence of blinding can also occur as an RCD ages. The RCD can become blind to various types of faults and the trip level may shift upwards resulting in blinding or reduced sensitivity resulting in less protection for users.

High frequency conditions on an installation can also be a cause of blinding. Taking the example of an inverter or motor controller installed on an installation, where there is an insulation breakdown to ground. We could get a fault current in the region of hundreds or possibly thousands of hertz. This can have quite an impact on conventional RCD designs where one of two things may happen. The RCD may nuisance trip or the RCD may be blinded.

Any event that causes blinding of an RCD is a huge risk to the user and in most instances the user will be totally unaware of the blinding. To deal with blinding it is becoming more evident that the concept of Self Testing and End of Life functionality should be integrated into RCDs because a blinded RCD will fail the Self-Test and switch to the End of Life state.