14/1/98
Page 25
Issue H
For 5A secondary
CT class 5P10 or 10P10
2.5VA
(Allowing for up to 0.08
Ω
of secondary lead resistance)
The CT primary rating must be not less than motor full load current and is usually selected as the
next highest standard ratio.
For Core Balance CT (CBCT)
CT class 1.0/5P5, 2.5VA
Note: higher standard VA ratings than suggested above may be chosen and this may allow higher
lead resistance and/ or improved protection performance.
Stabilising Resistor
In the case where the earth fault input is supplied from the residual connection of 3 line CT’s, it may
be necessary to fit an external stabilising resistor as shown on the circuit diagram. Guidance on
selecting a suitable resistor is given in P&B Engineering publication ref: MR902.
9.2. Testing.
The
MPR 3E5
Motor protection relay is of solid state design, and has a very low demand on
maintenance and testing. However it is recommended that Initial commissioning tests should be
completed and wherever possible should be made by PRIMARY CURRENT INJECTION, the main
objective being to prove the CT ratio and polarity and also the integrity of the CT secondary circuits.
This can then be followed by secondary injection tests to prove all the relay functions.
For commissioning and subsequent routine testing of the relay in situ, although not essential, it is
recommended that a test socket type MMLG be installed on the panel. This enables testing of the
relay without unplugging or disturbing the fixed wiring. Alternatively, a portable relay testing case is
available for testing the relay when withdrawn from the fixed portion.
NOTE.
If testing is carried out and the CTs are residually connected, the earth fault element must be disabled
during thermal/overcurrent testing to inhibit its operation.
In addition, if single phase testing is being used then the unbalance (NPS) function must be disabled
during all other tests. Refer to Trip Relay Matrix Tnnn.
Secondary Current Injection Testing.
•
By use of a secondary current injection single phase test unit, inject a variable current into the
relay, terminals 27 & 28. for Earth Fault. For the Overcurrent/thermal the phase inputs are wired
in series i.e. connect terminal 22 to 23, 24 to 25, 21 and terminal 26 are then connected to the test
current output.
•
Adjust the FLC setting to the calculated set point or alternatively 50% In.
•
Increase the injected current until the relay picks up, this measured current should be as set
above, or 0.5A,( 1A Relay) 2.5A,( 5A Relay). If the 50% figure is used.
•
It is also possible to carry out a timing test, by injecting various multiples of the setting current.
The expected trip times have been calculated for selected multiples in the table below.
