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GE Power Management MDP Digital Time Overcurrent Relay 2

GEK-100682D 2 OPERATING PRINCIPLES

2 OPERATING PRINCIPLES 2.1 INPUTS

The secondary current from the protected line current transformers is connected to the relay and immediately

reduced via the internal current transformers. The MDP CT secondaries are connected across input resistors,

yielding a voltage proportional to the relay input current. This voltage is rectified and filtered before being

applied to the multiplexer and analog-to-digital converter, which are internal to the microprocessor.

2.2 EXTERNAL INPUTS

Three external inputs (CP4, CP6, and CP7) are provided.

• CP4 – Block IOC Trip: when energized, blocks tripping by the ground units.

• CP6 – Block Ground Trip: blocks the phase and ground instantaneous trip elements when it is energized.

• CP7 – Breaker Status Input: when energized, indicates that the Breaker Status is "closed".

Since diode bridges are provided on all inputs, they can all be actuated by either switched-to-positive or

switched-to-negative, depending on the polarity connected to the stud. These inputs can be energized by

either AC or DC.

The Breaker Status and Block Ground inputs, CP7 and CP6 respectively, must be switched by the

same polarity, since they share a common ground (CP8).

2.3 COMMUNICATIONS PORT

The MDP relay has a communications feature that permits easy installation of various communications inter-

face modules. The procedure for installation/replacement of a communications upgrade card is provided

below:

1. To install or replace a communications upgrade card, begin by removing the relay from the case.

2. Before continuing, connect an approved wrist grounding strap to ground. This discharges any static elec-

tricity in your body that could damage the electronics of the MDP relay. A wrist strap is provided with the

communications card kit. The wrist strap must provide a "safe" high resistance to ground.

3. Remove four screws attaching the nameplate settings board and the NOMEX insulation paper to the relay

cradle (see the cover photograph). Note that there are four metal spacers between the nameplate and the

settings board. Be careful not to lose them or let them fall into the relay assembly. Carefully place the set-

tings board on top of the cradle to expose the communication card SIMM socket located on the lowest

board (digital input/communications board).

4. To expose the SIMM socket located on the lowest board, carefully push down on the board to disengage

the board spacers. Do not try to remove the board from the cradle

5. This step is for replacement only – skip to step 6 if you are installing the card for the first time. If you are

replacing an existing SIMM card, the old one must be removed! Start by simultaneously pushing the

catches on either side of the SIMM board outwards while lifting the board to a 45° angle. This will release

the board.

NOTE

Courtesy of NationalSwitchgear.com

GE MDP User Manual

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