Residual current is the current in a low voltage distribution circuit whose vector sum across all phases, including the neutral, is not zero. Clause 3.2.3 of GB 16917.1, Residual current operated circuit-breakers with integral overcurrent protection for household and similar uses (RCCB), Part 1: General rules, defines it as the vector sum, expressed as an RMS value, of the instantaneous currents flowing in the main circuit of the RCCB.

In plain terms, when a fault occurs on the load side so that the currents in the incoming and outgoing live and neutral conductors are no longer equal, the effective value of the instantaneous vector sum is the residual current. This current may flow from a live part through a human body to earth, which is commonly called leakage.

Residual current arises mainly because wiring in buildings is old, badly installed or affected by extreme weather, so insulation ages and cracks and part of the current leaks directly into the ground. A person touching such wiring may receive an electric shock. Because residual current protection used to appear only in distribution applications, it was a fairly specialised topic with little public exposure and the concept was rarely mentioned. In recent years, however, the fast growing field of electric vehicle charging piles and charging connectors requires users to draw power directly from distribution boards, and residual current protection has come back into focus.

Residual current devices are classified as type AC, type A, type F and type B, each adding functions on top of the previous one and each suited to different applications.

Type AC residual current

A type AC residual current device ensures tripping only for residual sinusoidal alternating current applied suddenly or rising slowly. This type of residual current has existed since alternating current was invented nearly two centuries ago. It is a typical sinusoidal alternating current, so its waveform is a sine wave. Even today, type AC residual current products account for more than 98% of the Chinese market.

Type AC residual current waveform

Type A residual current

As technology has advanced and living standards have risen, electrical equipment has become more complex. Household appliances now rarely use the heavy, expensive mains frequency transformers of the past. Instead most first obtain a pulsating DC voltage through a rectifier, then step down and filter it to supply the controller - dimmers and fan speed controllers, for example, very often use phase chopping to produce pulsating direct current. With the widespread use of equipment such as variable frequency drives for pumps, fans, air conditioners, cranes and lifts, and DC equipment such as welding machines, chargers, telecom equipment and DC motors, non-sinusoidal alternating residual currents have become more common in electrical circuits. Type A residual current devices were developed for these non-sinusoidal cases, adding protection against pulsating DC current to the type AC function. Typical type A residual current waveforms include A+0°, A-0°, A+90°, A-90°, A+135° and A-135°.

A+0° / A-0°

A+90° / A-90°

A+135° / A-135°

There is also a type A residual current waveform formed by pulsating DC superimposed on smooth DC, as shown below.

A+0° with DC +6 mA / A-0° with DC -6 mA

Type F residual current

Type A residual current products are now rapidly replacing conventional type AC. Schneider Electric has announced a full type A range at the same price as type AC, ABB and Siemens have followed, and many domestic residual current manufacturers are following suit. To capture the high ground, some companies have quietly launched type F products. Type F residual current technology adds detection of mixed high and low frequency waveforms to the type A function and raises immunity requirements, suiting a wider range of applications such as welding machines, lifts, inverters and airport lighting. The new waveforms include the typical type F composite wave and pulsating DC with 10 mA DC superimposed.

Type F composite wave (10 Hz, 50 Hz and 1,000 Hz combined)

A+0° with DC +10 mA / A-0° with DC -10 mA

Type B residual current

Type B is the highest grade of residual current device. It provides protection against sinusoidal alternating residual current up to 1,000 Hz, pulsating DC residual current, composite residual current and smooth DC residual current. For charging piles and charging connectors, which are closely tied to personal safety, type B protection is generally required. Type B devices represent the most comprehensive leakage protection technology available, and their high price has limited market uptake. With the rapid growth of electric vehicles in recent years and rising demand for charging piles, however, demand for type B products is increasing and prices are falling quickly. The additional type B waveforms include sinusoidal alternating current at 1,000 Hz and below but above 50 or 60 Hz, alternating current with smooth DC superimposed, pulsating DC with smooth DC superimposed, pulsating DC from two-pulse bridge rectification, pulsating DC from three-pulse star or six-pulse bridge circuits, and smooth DC.

AC 150 Hz / AC 400 Hz

AC 1,000 Hz

AC with positive smooth DC superimposed / AC with negative smooth DC superimposed

A+0° with positive smooth DC superimposed / A-0° with negative smooth DC superimposed

Positive pulsating DC from six-pulse bridge rectification / negative pulsating DC from six-pulse bridge rectification

Positive smooth DC / negative smooth DC

Charging infrastructure safety

As environmental awareness grows worldwide and the new energy vehicle market develops rapidly, electric vehicles have become an important direction for the automotive industry. With the growth of the EV fleet, however, the safety and reliability of the charging infrastructure that goes with it have become more prominent. To ensure the quality of charging equipment and the safety of users, the Chinese government has brought electric vehicle power supply equipment into the scope of China Compulsory Certification (CCC). From 1 August 2026, electric vehicle power supply equipment without a CCC certificate and certification mark may not leave the factory, be sold, be imported or be used in other business activities.

NTEK capability

Residual current device (RCD) testing is one of the key test items for charging piles, and NTEK Group already has the capability to test type B residual current protection. With strong technical and equipment resources, NTEK Group is ready to help companies improve product quality and move the new energy industry forward in a healthier and better ordered way.