zappi 6mA DC protection to IEC 62955

Summary

The myenergi zappi contains built in protection to trip the output if a residual current of 6mA DC is detected.   This protection is provided in accordance with IEC 62955:2018 [1] which refers to a general class of protective device, the RDC-DD or “Residual Direct Current Detecting Device”

The standard requires that the operation of the protection be tested in the presence of:

1. The sudden appearance of direct current (clause 9.9.2.1)
2. The steady increase of a smooth DC component (clause 9.9.2.2) (Ramped DC)

The requirements of the ramped DC test are ambiguous and this has led to a difference in the results obtained when testing an RDC-DD with test sets from different manufacturers.   This has led to Metrel, one of the leading test equipment manufacturers, issuing a software update to ensure consistent test results[2].

Performance requirements

IEC 62955 specifies a number of key performance requirements:

• Rated DC Residual Operating Current (I_Δdc)
  Defined in clause 5.3.2 as 6mA

• Rated DC Residual Non-Operating Current (I_Δndc)
  Defined in clause 5.3.3 as 0.5 I_Δdc = 3mA

• Limiting values of break time at sudden appearance of direct current
  Defined in clause 5.3.10.1 and table 2

In accordance with these requirements, zappi is designed to trip within 900ms once the measured value of residual DC current exceeds 5.5mA

zappi protection performance - steadily increasing DC component

Clause 9.9.2 states:

This clause is ambiguous and the measured values obtained will depend very heavily on the ramp rate selected by the test equipment manufacture.

Consider the case where the ramp rate is set to increase at 5mA / second and the protection operates at 5.5mA

Starting at 2mA, the DC current exceed the 5.5mA upper threshold after 700ms.
If the protection operates once the current exceed 5.5mA and trips within 1 seconds then the current at the moment the protection operates will be 10.5mA

Clause 9.9.2.1 states that the “(residual current) trying to attain the value of 6mA with 30s.   Starting at 2mA, a ramp rate of 0.133mA per second if required to obtain the 6mA value 30 seconds after starting the test.

Now consider the case that the test set ramps the DC current at a rate of 0.133mA per second.

For zappi, the trip threshold (5.5mA) is reached after 26.25 seconds.

Zappi trips one second later when the residual DC current will be 5.6mA which is within the 6mA limit specified for this test.

It is clear therefore that the test depends heavily on the ramp rate of the test set which, to avoid false failures, must be set to ramp at a rate of 0.133mA per second. 

zappi Test Results

Serial Number: CTB18085451 

When tested with the Metrel MI 3152 all tests pass.  The Metrel MI 3152 has had the software update that was issued specifically to deal with the IEC 62955 DC ramp rate test

Tests carried out with the Megger MFT1741+ fail as this tester does not respect the ramp rate restrictions for the DC ramp rate test

If your Megger MFT1741+ is on firmware LESS than V4.02 then please get in touch with the Megger Customer Experience Team who can arrange for your firmware to be updated alongside a standard calibration.  The Megger team will confirm the cost of the calibration when you contact them. 

[1] Residual direct current detecting device (RDC-DD) to be used for mode 3 charging of electric vehicles [2] https://www.tester.co.uk/blog/electrical/metrel-mi3152-multifunction-tester-ev-firmware-update/