I-GARD DSP-OHMNI Instruction Manual
4
The Feeder Modules measure the fault current level in the branch circuits that are protected. This module
uses standard I-Gard current sensors Type T2A, T3A, T6A and T9A. It is equipped with a form C 10A
output Relay that can be used for breaker control. The DSP-DFM detects two fault levels. Firstly it detects
the single fault, which creates a System Alarm condition, and secondly through a priority level system it
provides breaker control to disconnect the least important circuit breaker.
Communications is provided by a 4-wire RS-485 network connection from a jack located at the rear of the
DSP-DM Display module. The communications protocol supported is MODBUS RTU, which is a master/
slave system with selectable baud rates from 4800 to 19200. The DSP supports the MODBUS function Read
Holding Registers only, without exception support. Additionally it will support remote RESET using the Force
Coil function.
2
APPLICATION
The DSP system is used in conjunction with I-Gard, Alarm Resistor Unit Type DDR2. The DDR2 matches
the DSP-DSM input circuits to the system voltage and is available in 4 types as follows:
Type
System Voltage
DDR2-1
120V*
DDR2-2
240V
DDR2-4
480V
DDR2-6
600V
* Also used with potential transformers up to 13.8KV
The DDR2 provides output voltages V
AG
, V
BG
, V
CG
that are proportional to the phase to ground voltage and
also voltage V
NG
that is proportional to the neutral resistor voltage. ( i.e. Total leakage/fault current of the
system)
On large systems provision is usually made to ground the system using a current-limiting resistance
(I
-Gard Type OHMNI-PM or NGR). On ungrounded systems there is always leakage capacitance to ground
from each line. Re-striking ground faults may cause an excessive build up of line to ground voltage due to
this capacitance. It may be stabilized with the addition of a grounding resistance, thus preventing costly
breakdown of insulation.
The Type OHMNI-PM is connected between ground and the star point of the transformer on Wye systems.
On Delta systems an artificial neutral device (I-Gard Type DDAI) is required to provide a star point. Both
OHMNI-PM and DDAI devices are selected for appropriate current ‘let-through’
,
i.e.: The current, which will
flow to ground, if there is a direct short from line to ground (on any one phase).
NOTE:
A good Rule-of-Thumb for Resistor current selection is 1 ampere per 2000KVA, if no surge capacitors are on the system,
and 1 ampere per 1000KVA with surge capacitors. For further information please refer to www.i-gard.com/appguides.htm
