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Residual Current Protective Device

General

A RCD protects from an existing voltage (not from developing) for an inadmissibly high contact voltage and therefore avoids efficiently dangerous or even fatal injuries in current accidents and avoids fire as well.

A Residual Current Device (RCD) is a protection device in power supply networks. In Europe RCDs are usually installed in addition to over-current protection devices (fuses), in Germany serial fuses are mandatory.

In germany pulse current sensitive RCD types A are usual, which detect sine AC current and pulsing DC currents as well. Pulse current sensitive RCDs are independent from the mains voltage level.

RCD types B (all current sensitive) detect the smooth DC current with a second sum current converter. Internal electronics triggers the RDC in case of an error. The DC fault current detection depends on the mains voltage level.

Function

The RCD separates the supervised circuit with all poles (except the protective earth) from the mains, if a specific differential current (usually 30mA in house applications) is exceeded.

Differential currents can occur, e.g. if a fault current flows through a human body or through a defective isolation. To detect this, the RCD compares the level of input current with the flowing back current. In an error free application the sum of both current must be zero.

If B6 rectifier is used with a standard RCD (e.g. in servo amplifers) and a fault current occurs behind the rectifier, then the RCD is pre-magnetized by the DC fault current and therefore inactive. The RCD is saturated and cannot detect any fault in the circuit. That's why only RCDs type B (all current sensitive, with electronics) are still functional in theis case and therefore permissible.

Use with servo amplifiers

In conformity with IEC 60364-4-41 – Regulations for installation and EN 60204 – Electrical equipment of machinery, residual current protective devices (called RCD) can be used provided the requisite regulations are complied with.
The RCD cannot differentiate fault current and leakage current and therefore measures them both. So switch off can occur if the sum of all leakage currents exceeds the switch off level, although there is no fault current in the application.
The servo amplifier is a 3-phase system with a B6 bridge. Therefore, RCDs which are sensitive to all currents must be used in order to detect any D.C. fault current.

Fault Current

Fault currents are usually ohm's currents and appear because of isolation faults between voltage leading parts and ground (earth), caused by e.g.  dirt or humidity.

Leakage current

Leakage currents are usually capacitive currents and flow because of EMC measures through capacitors in EMC filters or the capacity of long, shielded cables to ground (earth). As a rule of thumb, the following assumption can be made for leakage current on low-capacity cables at a mains voltage of 400 V, depending on the clock frequency of the output stage:

  •  Ileak = n x 20mA + L x 1mA/m at 8kHz clock frequency at the output stage
  •  Ileak = n x 20mA + L x 2mA/m at a 16kHz clock frequency at the output stage
    (where Ileak=leakage current, n=number of amplifiers, L=length of motor cable)
    At other mains voltage ratings, the leakage current varies in proportion to the voltage.
  • Example: 2 x servo amplifiers + a 25m motor cable at a clock frequency of 8kHz:
    2 x 20mA + 25m x 1mA/m = 65mA leakage current.

Since the leakage current to PE is more than 3.5 mA, in compliance with IEC61800-5-1 the PE connection must either be doubled or a connecting cable with a cross-section >10mm² must be used. Use the PE terminals and the PE bolt in order to fulfil this requirement.

Stationary Leakage Current

Caused by asymmetrical mains voltage, which causes a capacitive current in symmetrical mains filters, that flows to ground via the protective earth line. The stationary leakage current exists even with stand still motor, has typically a frequency of  100Hz to 1kHz and an amplitude of several 100mAmps.
Possible reduction by:

  • use EMC filters with low leakage current
  • use four lines filters in case of mains with neutral line
  • distribute several single phase operated servo amplifiers evenly to the phases
Variable Leakage Current

Caused by speed control of motors and by the switching frequency of the amplifier's power stage. Amplitude and frequency (> 1kHz) depend on the operating situation.
Possible reduction by:

  • shielded cables as short as possible
  • install sine filter or choke to the output of the servo amplifier
  • install additional four lines filter in case of a system with several servo amplifiers with integrated filters
Transient Leakage Current

Caused by switch off situations of inductions in the mains that produces voltage peaks on the lines. Switching on devices during an unfavorable phase angle of the mains voltage leads to steep voltage  peaks, too, and therefore to short term high frequency portions. These high frequency signals are led through the capacitors in the EMC filters to ground (earth) and can trigger the RCD.

Rated residual currents in the RCD

  • Leakage current 10 -30 mA
    Protection against "indirect contact" (personal fire protection) for stationary and mobile equipment, as well as for "direct contact".
  • Leakage current 50 -300 mA
    Protection against "indirect contact" (personal fire protection) for stationary equipment

Recommendation: In order to protect against direct contact (with motor cables shorter than 5 m) we recommend that each servo amplifier be protected individually using a 30mA residual current protective device which is sensitive to all currents. If you use a selective RCD, the more intelligent evaluation process will prevent spurious tripping of the RCDs.