Ampcontrol Pty Ltd
– ABN 28 000 915 542
ELV-PRO USER MANUAL
MAG-219 Version 5
– NOVEMBER/2020
Uncontrolled Copy - Refer to Ampcontrol Website for Latest Version
Page 47 of 59
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A2 Improving Protection
The design of earth leakage relays used in mining applications in Australia and New Zealand must comply with
AS/NZS 2081. The latest version of this standard (AS/NZS 2081:2011) better recognises that system protection
needs to be assessed in accordance with AS/NZS 4871.1:2010 and with the changes in the mining electrical
environment.
The following extracts from AS/NZS 2081:2011 provide some key statements of interest:
1.1 Scope
Whereas this standard is based upon 50 Hz supply systems, it is envisaged that the equipment
described may also be installed in systems with higher, lower or variable frequencies, or in dc supplied
systems.
AS/NZS 60479, Part 1 and 2 should be referenced for consideration of the effects at other supply
frequencies upon the human body.
Appendix B:
The diversity of operating conditions and equipment addressed by this Standard precludes reliance
solely on explicitly prescribed trip levels or fault current levels, and their duration, in order to ensure a
safe working environment. Rather, the onus is placed on the system designer to ensure appropriate
touch voltage/operating times when integrating the protection devices addressed by this Standard.
B2 Voltage/duration Thresholds
Design criteria for the protection devices have been chosen to enable compliance to the touch
voltage/operating times for systems operating at 50 Hz as described by Figure B1.
B3 Systems at other than 50 Hz Cyclic Frequency
Where equipment is installed and operated within systems at other than a constant 50 Hz cyclic
frequency, the characteristics in paragraph B2 are not immediately applicable. In such instances,
individual calculation to determine requirements at the frequency or frequencies in question will be
required.
Standards AS/NZS 60479.1 and AS/NZS 60479.2 should be referenced in relation to the effects upon
the human body of other supply frequencies.
What this means is that when VSDs (or other non 50Hz sources) are used in a mining electrical system then the
standard approach needs to be interpreted to ensure that protection is adequate. The key factors to consider are
as follows:
1) The sensitivity of the human body to electric shock varies with frequency. In general, for a given exposure
time
, the allowable touch voltage magnitude increases with frequency. For example, at 10 kHz, the “let go”
voltage is about 5 times that level at 50 Hz.
2) When EMC filters are used, this forms a path for earth currents alternative to the NER. When considering
touch potentials at a mobile machine for example, strictly speaking the impedance of the filter at the
frequency of interest should be examined in order to determine the earth fault current that will flow when a
fault occurs in the machine. The earth leakage trip time must then be used to ensure that the touch voltage
and exposure time guarantee a safe system. Care must be taken when multiple filters are connected to a
single supply, as this presents many modes of possible earth fault that need to be considered and actual
earth fault currents may exceed the current seen by any single filter.
3) An earth leakage relay must be able to accurately sense earth fault currents of any frequency from dc to
the maximum frequency of interest.
It can be seen that this is not a trivial matter and it is likely that the industry will need to adapt to this new and
complex environment.