Ground Fault Sensor and GFCI

GROUND FAULT DETECTION

Ground faults occur when current flows from an energized conductor to ground, which can energize other equipment and potentially cause harm if touched. Ground fault detection is critical to protect people and animals from being shocked or killed. IVY Metering, a leading supplier of current sensing and power monitoring products, offers a broad selection of ground fault sensors used to prevent harm to personnel, equipment, and processes.

Why is Detecting Ground Faults Important?

Ground faults can be costly if not checked. They can cause a fire to erupt, create equipment damage, or even shock or electrocute personnel. Numerous safety regulations and electrical codes exist to prevent and protect against ground faults. 

What is a Ground Fault Sensor?

Ground fault sensors consist of a single magnetically permeable toroid surrounding current carrying conductors connected to an electrical device such as a motor, heater element, pump, or other machinery. The current transformer detects any imbalance through the conductors measuring current leakage to ground. The ground fault sensor may then trigger a contactor to disconnect power or to send an alarm to a higher-level control that initiates the appropriate action.

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 person’s 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.

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.