I-Gard DSP-OHMNI, Manual

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1. INTRODUCTION
High resistance grounding (HRG) is becoming more prevalent in industrial and commercial electrical power
systems. As the need for reliable and stable power increases, the inconvenience of unwanted downtime in
processing, robotics and data service also become more critical and costly.
Single ground faults in motors and equipment are common and will cause interruption of service in solidly grounded
systems. HRG offers service continuity by limiting the ground fault current to a sustainable level for an indefinite
time.
I-Gard’s DSP-OHMNI is an HRG system designed to provide service continuity using a Neutral Grounding Resistor,
a voltage alarm unit and a ground fault relay to detect the event of a single ground fault, signal an alarm condition
and display the affected phase and/or feeder using programmable voltage and current measurements.
The DSP-OHMNI system also allows the operator to identify the precise fault location using an integrated pulsing
module available in the OHMNI-PM or OHMNI-PM-RM resistor to modulate the current in the fault circuit which
can be detected using a portable clamp-on current probe (TS-SENSOR) connected to an ordinary ammeter (DMM)
meter. Thus, maintenance can be immediately alerted and an operator dispatched to locate the fault to isolate it
promptly.
In some applications, however, it is desirable to clear any fault on the system when it occurs with fire prevention
or protection of sensitive equipment being the main concern rather than continuity of service. e.g. petro-chemical,
grain handling. In this case, in addition to fault indication, the DSP-OHMNI can be programmed to trip the breakers
associated with the fault with or without adjustable time delay (up to 99 hours). The alarm level is normally set at
50% (default) of the system maximum ground current, however with the DSP-OHMNI system other alarm levels
between 10% and 90% may be set as desired.
Additionally, the DSP-OHMNI has two TRIP modes. It can be set up to control circuit breakers to TRIP on the
occurrence of a single fault with or without time delay, or if the feeder circuit ground fault current exceeds 100A
on two (phase to ground to phase) faults (this can only occur when two faults exist since the current on a single
fault cannot exceed the grounding resistor rated current). The DSP-OHMNI can then be set-up to selective second
ground fault trip on a priority basis to protect the critical feeder. Because two circuits will be involved it is possible
to prioritize which circuit will be interrupted by a priority setting. This ensures that only the least important circuit is
interrupted.
The DSP-OHMNI system consists of a voltage sensor unit (DDR2), a Neutral Grounding Resistor (OHMNI-PM or
OHMNI-PM-RM) and a ground fault relay (DSP-DSYS) which includes a panel mounted display (DSP-DM) and
two 35mm DIN rail mounted modules (DSP-DPS & DSP-DSM) typically located inside the control compartment of
a switchgear. The modules are connected together using 20 pins-conductor standard ribbon cables. The panel-
mounted display module (DSP-TDM) is a Human Interface Device (HID) that allows set-up and control of the
system. Optional 35mm Din Rail mountable modules are available for specific applications.