Ground Fault Circuit Interrupter(GFCI) and Residual Current Device(RCD)

A GFCI (ground fault circuit interrupter) is intended to provide protection against electric shock. It does this by opening one or more contacts to disconnect power from a circuit or load when the ground fault current fl owing in the circuit protected by the GFCI reaches the rated operating current of the GFCI.

In the case of GFCIs, a ground fault current therefore refers to any current other than the load current. The ground fault current may fl ow through a persons body or through a fault in equipment or wiring anywhere on the load side of the GFCI. The key factor in the ability of the GFCI to detect a ground fault current and provide protection is the connection of the supply neutral conductor to earth at the origin of the installation.

Although the load current will typically be in the amperes range, the differential current In will generally be in the milliamps range. Typical maximum values of In in the USA are 6mA for personal shock protection, 30mA for equipment protection, and 300mA for fi re protection.

It is worth noting that the normal human body resistance for most people is in the range 1000 - 2000 Ohms, but for some people it may be as low as 500 ohms, so for a touch voltage of 120V the current through the body will generally be in excess of 50mA.

6mA versus 30mA Operating Thresholds

In the USA, the operating threshold for a GFCI for personal shock protection is 6mA. In most of the rest of the world, the threshold is 30mA. There are two reasons for this difference. The first relates to safety, and the second relates to technology.

Tests on the human body have indicated that as the current is increased from zero mA, muscles tend to seize at about the 10mA level with the result that a person touching a live conductor may not be able to let go a live conductor at currents above 10mA. This is referred to as the let-go limit, and the North American GFCI standards decided to set the operating level of GFCIs at 6mA so as to have a comfortable safety margin below this threshold. At about 40mA, heart fi brillation starts to occur, which could lead to heart failure. European manufacturers decided to set 30mA as the operating level of RCDs (GFCIs) so as to have a comfortable safety margin below this threshold. This begs the question as to why the Europeans would set a higher level than the let-go level. The answer is Technology. North America uses electronic technology in their GFCIs, and it is relatively easy to set an operating threshold of 6mA for such devices. However, most European devices use electromechanical technology, and it is extremely diffi cult for such devices to operate at the 6mA level, so they chose 30mA because they can detect this level quite easily and it provides protection against heart fi brillation. European manufacturers had a substantially greater infl uence on IEC standards and practice, and the 30mA level has been adopted in most areas worldwide with the exception of North America.

DC Sensing GFCIs

DC ground fault currents can occur in installations powered from DC, such as Mines Tunnels Solar panels Electric vehicles Ships Aircraft, etc.

DC currents through the body can be every bit as dangerous as AC currents and GFCI protection should be provided where shock risks exist. In IEC, GFCIs/RCDs that provide protection against both AC and DC ground fault currents are referred to as B Types Comm.

A residual current is any current flowing to earth (ground) through a person or equipment that could present  a shock risk or a fire risk.

A Residual Current Device (RCD) is a device intended to detect dangerous residual currents and thereby mitigate the risk of electric shock or fire.

A Type B RCCB can detect AC and DC residual currents, and is ideally suited for use in EV charging. However, Type B RCCB can be relatively expensive. A new type of RCD has emerged recently, known as an Type B MD series RCD sensor. This can also detect AC and DC residual currents, but has been optimised for use in EV charging and is a lower cost alternative to a Type B RCCB.

In the case of EV charging, there is a risk of AC and DC residual currents, and it is vitally important that the MD series RCD are used to detect AC and DC residual currents.