6 EARTH PROTECTION FUNCTIONS
6.1 Earth Leakage
The Earth Leakage protection function uses a toroid to measure the earth leakage current. A definite
time operating characteristic is provided with an adjustable trip sensitivity and time delay.
When a fault occurs and the trip level and time delay are exceeded; a trip occurs. The trip acts in the
Main Contactor Relay (MCR) logic and is latched. An earth leakage trip is treated as a special fault and
requires an authorised person to perform the reset function. This is achieved by holding the lock input
closed and then closing the reset button.
When an earth leakage trip occurs, the “EL” LED on the display module flashes and the open collector
output on the HPB Relay is switched on the provide additional monitoring if required.
The measured instantaneous leakage current (EL) is displayed on the HDM “Earth Fault Information”
page as a percentage of the trip level. When the leakage reaches 100% for the selected time delay a trip
occurs.
For the HPB 6.6kV and the HPB 22kV 200mA EL, the trip level is adjustable in 100mA increments over
the range 200mA-1000mA and for the standard HPB 22kV the trip level is selectable in 250mA
increments over the range 500mA-2500mA. The time delay is selectable as instantaneous (<80ms) or
adjustable in 40ms increments over the range of 150ms-470ms.
6.2 Earth Fault Lockout
6.2.1 HPB 6.6kV
On 6.6kV systems the earth fault lockout function tests the resistance of the 3 phase lines to earth by
applying a “megger test” prior to closing the main contactor. The test is initiated by closing the start input,
provided all other starting conditions are met (see Section 10.5, Operational Sequence). The HPB Relay
closes its relay output, RL3, which applies 100VAC to the CCMB-6.6kV (Cable Connection Module type
B – 6.6kV). This is a resistive isolation device used to interface the HPB Relay to the power conductors.
A 5kVDC voltage is generated within the CCMB-6.6kV, which under normal operation applies up to
2.5kVDC between each phase and earth.
The HPB Relay measures the voltage on the line and calculates the meg-ohm resistance to earth of
each phase. At the end of the test, provided the value is above the pre-set threshold, the MCR relay
closes allowing the outlet to be energised. If the value is below the pre-set threshold, an Earth Fault Trip
occurs.
At the completion of a test, the resistance to earth of each phase is retained in memory until the next test
is carried out. This can be viewed on the HDM (Remote Display Module) (level3, position 2).
6.2.2 HPB 22kV
On 11kV and 22kV systems the earth fault lockout function tests the resistance of the 3 phase lines to
earth by applying a “megger test” prior to closing the main contactor. The test is initiated by closing the
start input (provided all other starting conditions are met (see Section 10.5, Operational Sequence). The
HPB Relay closes its relay output, RL3, which applies 48VDC to the CCMB-22kV (Cable Connecting
Module type B – 22kV). The CCMB-22kV then disconnects the primary windings of its 3 x voltage
transformers from earth (they are connected to earth at all other times), and generates 5kVDC which is
applied to the primary star point, resulting in 5kVDC between all three phases and earth. The HPB Relay
then measures the current to earth and calculates the resistance of the line insulation to earth of all three
phases in parallel. Three seconds (3s) after the end of the test, if the calculated resistance is above the
present threshold, the MCR relay closes allowing the outlet to be energised. If the calculated resistance
is below the pre-set threshold, an Earth Fault Trip occurs.
At the completion of a test, the result is retained in memory until the next test is carried out. This can be
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