S Type RCDs

An S Type RCD is a time delayed RCD that performs very differently to a typical RCD.

Generally, the job of an RCD is to remove power very quickly under fault conditions, hence protecting against electric shock.

An S Type RCD is intended to operate more slowly so as to minimise the risk of nuisance tripping. However, it is important to note that S Type RCDs are not suitable for personal shock protection because they may allow current to flow through the body for an unacceptably long period. Therefore S Type RCDs should not be used to provide personal protection. Their primary function is to provide protection against electrical fire. In many industrial type installations where the main concern is not shock risk but rather fire risk, S Type RCDs are perfectly suited.

Ignition of a fire by an electrical fault current requires a combination of current and time and S Type RCDs are designed with a current/time characteristic built in that ensures that an electrical fire will not occur within the response time of the RCD. The typical response time for a 100ma S Type RCD is in the region of 200 milliseconds for a current of 200 milliamps.