Differential current device

UDTs with a rated tripping differential current not exceeding 30 mA are intended for additional protection of a person from electric shock. It is used as part of the protection “automatic power off” ^[3] .

In alternating current systems, additional protection through UDT should be provided for:

• plug and power sockets with a rated current of up to 32 A;
• mobile equipment with a rated current of up to 32 A, which is used outdoors.

UDT disconnects the protected circuit:

• with the direct contact of a person or animal with electrical equipment under voltage;
• in case of damage to the main insulation and contact of live parts with an open conductive part.

Requirements for the installation and use of UDT are given in a series of standards for electrical installations of buildings IEC 60364.

The main component of the UDT is a differential transformer, which is designed to detect differential current. If the differential current exceeds the value of the tripping differential current or is equal to it, the electric circuit will open.

The photo shows the internal structure of one of the types of UDT. This UDT is intended for installation in a wire break. Linear and neutral conductors from the power source are connected to the contacts (1), the main circuit of the UDT is connected to the contacts (2).

When the button (3) is pressed, the contacts (4) (as well as another contact hidden behind the node (5)) are closed, and the UDT passes current. The solenoid (5) keeps the contacts closed after the button is released.

Secondary winding (6), to which a differential current release is connected. In the normal state, the current of the linear conductor is equal to the current of the neutral conductor, however, these currents are opposite in direction. Thus, the currents mutually cancel each other and there is no EMF in the coil of the differential transformer.

The earth fault current leads to an imbalance in the differential transformer: a larger current flows through the linear conductor than through the neutral conductor (part of the current flows through the human body, that is, bypassing the transformer). Differential current in the primary winding of a differential transformer leads to the appearance of an EMF in the secondary winding. This EMF is immediately registered by the tracking device (7), which turns off the power to the solenoid (5). The disconnected solenoid no longer holds the contacts (4) in the closed state, and they open under the action of the spring force.

The device is designed in such a way that the shutdown occurs in a split second, which significantly reduces the severity of the consequences of electric shock.

The test button (8) allows you to check the operability of the device by passing a small current through the orange test wire (9). The test wire passes through the core of the differential transformer, so the current in the test wire is equivalent to the imbalance of the current-carrying conductors, that is, the UDT should turn off when the test button is pressed. If the UDT does not turn off, then it is faulty and must be replaced.

The UDT will not work if the person is energized, but the earth fault current does not occur, for example, by touching both the linear and neutral conductors of the protected circuit. It is impossible to provide protection against such touches, since it is impossible to distinguish the flow of current through the human body from the normal flow of current in the load. In such cases, only mechanical protective measures ( insulation , non-conductive covers, etc.) are effective, as well as shutting down the electrical installation before servicing it.

The UDT, functionally dependent on the mains voltage, needs the power that it receives from the protected circuit. Therefore, a potentially dangerous situation is when a neutral conductor breaks above the UDT and the line conductor remains energized. In this case, the UDT will be unable to disconnect the circuit, since the voltage in the protected circuit is insufficient for functioning. The UDT is functionally independent of voltage and is free of this drawback.

The first patent (German patent No. 552678 dated 08.04.28) for UDT was obtained in 1928 by the German company RWE (Rheinisch - Westfälisches Elektrizitätswerk AG). The first valid prototype of the protective device was manufactured by the same company in 1937. A small differential transformer was used as a sensor, and a polarized relay with a sensitivity of 0.01 amperes and a speed of 0.1 s served as an actuator ^[4] .

The sensitivity of the prototype device was 80 mA ^{[5] a} further increase in sensitivity was inhibited by the absence of materials with the desired magnetic properties. In 1958, Dr. Beaglemeier from Austria proposed a new circuit design for the UDT. Now such UDTs are marked with the letter G. False positives from lightning discharges were eliminated in the design and sensitivity was increased to 30 mA ^[5] .

The boundary curves of alternating current and the physiological effect of current on the human body ^[6] were established by tests in the years 1940-1950 at the University of Berkeley by the American scientist Charles Dalzil. During the tests, volunteers were exposed to electric current with a known voltage and current strength ^[4] .

In the early 1970s, most UDTs were manufactured in circuit breaker- type housings. Since the early 1980s in the United States, most household UDTs have already been plugged into outlets .

In the USSR, the first experiments on the design of UDT began in 1964 ^[7] . The first serial UDT to equip a three-phase electrified instrument was manufactured in 1966 by the Vyborg Electric Tool Plant, developed by VNIISMI . The first household UDT in the USSR was developed in 1974, but it did not go into the series ^[8] . Serial household UDT has been produced since 1988 in significant quantities (up to 200 thousand units per year). A typical type of UDT of that time is an extension cord with a socket on the cord. Since 1982, all educational electrical equipment that went to schools was equipped with mandatory UDT, which received the name "school". The seriality of the product reached 60 thousand pieces per year. For the needs of industry and agriculture, protection IE-9801, IE-9813, UZOSh 10.2 (still in production), RUD-0.5 were produced.

Currently, UDTs are mainly used for installation in an electrical panel on a DIN rail , and built-in UDTs are not yet widely used.

By way of management

• UDT without auxiliary power supply
• UDT with auxiliary power supply:
  • performing automatic shutdown in case of failure of the auxiliary source with time delay and without it:
    • producing automatic restart when restoring auxiliary source
    • not automatically restarting when restoring auxiliary source
  • not producing automatic shutdown in case of failure of the auxiliary source:
    • capable of shutting down in the event of a dangerous situation after failure of the auxiliary source
    • incapable of shutting down in the event of a dangerous situation after failure of the auxiliary source

By installation type

• stationary with installation stationary electrical wiring
• portable with mounting flexible wires with extension cords

By the number of poles

• bipolar;
• four-pole.

If possible, control the tripping differential current

• unregulated;
• adjustable:
  • with discrete regulation;
  • with smooth regulation.

Resistance to impulse voltage

• allowing the possibility of shutdown at a pulsed voltage;
• resistant to surge voltage.

According to the operating conditions in the presence of a DC component

AC type UDT : UDT, the operation of which is provided by a differential sinusoidal alternating current either by its sudden application, or with a slow rise ^[9] .

Type A UDT: UDT, the operation of which is provided by both a sinusoidal variable and a pulsating constant differential current through either a sudden application or a slow rise ^[9] .

UDT type B : UDT, which guarantees operation as a device of type A and additionally triggers:

• with differential sinusoidal alternating current frequency up to 1000 Hz;
• with differential sinusoidal alternating current superimposed on a smoothed direct current;
• with a pulsating differential direct current superimposed on a smoothed direct current;
• with a differential pulsating rectified current from two or more phases;
• with a differential smoothed direct current applied suddenly or gradually increasing, regardless of polarity ^[10] .

Type F UDT: A UDT that guarantees operation as a Type A device in accordance with the requirements of IEC 61008-1 and IEC 61009-1 and additionally operates:

• with a composite differential current applied suddenly or gradually increasing between phase and neutral or phases and the middle grounded conductor;
• with a pulsating differential direct current superimposed on a smoothed direct current ^[10] .

By the presence of a time delay (in the presence of a differential current)

• UDT without delay - type for general use;
• Time-delayed CTD - Type S for selectivity.

• Residual current circuit breaker with integrated overcurrent protection
• Grounding

• IEC / TR 60755: 2008. General requirements for residual current operated protective devices. Edition 2.0 - Geneva: IEC, 2008-01.
• IEC 60947-2: 2016. Low-voltage switchgear and controlgear. Part 2: Circuit-breakers. Edition 5.0. - Geneva: IEC, 2016‑06.
• IEC 61008-1: 2013. Residual current operated circuit-breakers without integral overcurrent protection for household and similar uses (RCCBs). Part 1: General rules. Edition 3.2. - Geneva: IEC, 2013‑09.
• IEC 61009-1: 2013. Residual current operated circuit-breakers with integral overcurrent protection for household and similar uses (RCBOs). Part 1: General rules. Edition 3.2. - Geneva: IEC, 2013‑09.
• IEC 61540: 1999. Electrical accessories. Portable residual current devices without integral overcurrent protection for household and similar use (PRCDs). Edition 1.1. - Geneva: IEC, 1999-03.
• IEC / TR 62350: 2006. Guidance for the correct use of residual current-operated protective devices (RCDs) for household and similar use. First edition. - Geneva: IEC, 2006‑12.
• IEC 62423: 2009. Type F and type B residual current operated circuit-breakers with and without integral overcurrent protection for household and similar uses. Edition 2.0. - Geneva: IEC, 2009-11.
• IEC 60050-442: 1998. International Electrotechnical Vocabulary. Part 442: Electrical accessories. Edition 1.0. - Geneva: IEC, 1998-11.
• GOST R IEC 60755-2012. General requirements for protective devices controlled by differential (residual) current.
• GOST IEC 61009-1-2014. Residual current circuit breakers with built-in overload current protection, household and similar purpose. Part 1. General rules.
• GOST R 51328-99 (IEC 61540-97). Residual current circuit breakers for household and similar purposes, controlled by differential current, without built-in protection against overcurrents (UZO-DP). General requirements and test methods.
• Kharechko Yu. V. Protective devices of modular design. - M.: LLC “ABB Industry and Construction”, 2008. - 336 p.
• Harechko Yu. V. A brief terminological dictionary of low-voltage electrical installations. Part 4 // Appendix to the journal “Library of an engineer for labor protection”. - 2015. - No. 6. - 160 c.
• Gurevich V. I. False positives of RCD: who is to blame and what to do? / Vladimir Gurevich (Ph.D.) // Power Electronics. - 2013. - No. 5. - S. 48 - 54.

• Recommendations for the use of residual current circuit breakers