The disadvantages are:
a. An incorrect healthy Earth Continuity Indication could be produced by a pilot that has faulted to
earth with a resistance within the pickup range of the HPB Relay. For example if the selected trip
level is 50Ω, then the fault to earth in the range of 200Ω to 285Ω would be seen as a healthy
circuit.
b. The “Machine Communication” functions of the HPB Relay are not available.
The advantages of using HTU mode are:
a. Machine communication.
b. Machine stops can be differentiated from Earth Continuity Trips in the HPB Event Log.
c. Increased security in pilot system as the HPB Relay must receive an intelligent signal from the
HTU-1 Termination Module in order to deem the pilot circuit healthy.
The net disadvantage is that the increased complexity in the system may make troubleshooting difficult.
Resistor Mode
When the pilot is set to “Res” Mode the trailing cable pilot is terminated with a 235Ω resistor to earth (2 x
470Ω, 5 Watt resistors connected in parallel). The HPB Relay measures the resistance of the pilot earth
loop and assumes that the pilot circuit is healthy if the resistance measured is between 235Ω and 235Ω
+ RTrip (the selected trip resistance). If the resistance measured is below 200Ω a short circuit fault
between the pilot and earth conductors is assumed and a trip occurs, which in turn de-energises the
MCR Relay.
In “Res” mode all of the relay settings are stored in the HPB Relay. If a load is moved to a different outlet
(or substation) then the HPB Relay’s settings need to be checked and updated if necessary.
HTU Mode
When the pilot is set to “HTU” Mode the trailing cable pilot is terminated with a HTU-1 Termination
Module. The HPB Relay measures the resistance of the pilot earth loop and assumes that the pilot circuit
is healthy if the resistance measured if below the selected trip resistance level. If the resistance exceeds
the pre-set level a trip will occur. The Event Log will display “EC Ω Trip”
In “HTU” mode the relay settings that pertain to the actual load connected are stored in the HTU-1
Termination Module, which is installed in the machine. If a machine cable is moved to a different outlet
(or substation) then the protection settings for the machine are automatically uploaded to the HPB Relay.
The Earth Continuity Resistance (ECR) of the pilot – earth loop is displayed in ohms on the HDM “Earth
Fault Information” page (level 3, position 1). The leakage (SC) between the pilot and earth conductors is
displayed as OK or Trp. The Event Log will display “EC Short” if a short occurs between pilot and earth
when the outlet is energised.
The Earth Continuity (EC) Trip resistance for both modes is selectable to 50, 75, and 100 ohms.
Pilot Trip Time is adjustable to allow for operation in noisy electrical environments. The following trip
times are available: 300ms, 400ms, 500ms, 600ms, 800ms, 1.0s, 1.2s, 1.5s, and 2s.
A maximum of 600ms should be suitable for most installations.
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