Page 382 - Offshore Electrical Engineering Manual

Protection Relays 369

ELECTRICAL TESTS ON ATTRACTED ARMATURE RELAYS
Attracted armature relays are used as lockout, tripping, control, counting and indicat-
ing relays designed to sense variations in either system current or voltage. Such a
relay may be located either as a single unit in its own case or as one of a number of
relays within a switchboard cubicle.

Insulation Testing
With all secondary wiring disconnected from the relay, a 500 V DC insulation test
should be carried out on all terminals of the relay unit. The resistance value to earth
should exceed 10 MΩ.

Secondary Injection Testing
In general, voltage-operated relays should be tested to ensure that they operate
satisfactorily at 75%–120% of normal coil rated voltages for DC operation and
80%–115% for AC operation. Where voltage measuring relays are used, the pickup
and dropout voltages should be checked against calibrated values. (The dropout volt-
age is the voltage at which the relay coil is no longer able to hold the armature in its
energised position, so that it opens or drops out.)
Current-operated relays should be tested to ensure that the minimum operating
current does not exceed the manufacturer’s recommended value.
In the case of auxiliary relays, tapped coils are provided and the pickup current
should not exceed the value recommended for the particular tap. For current measur-
ing relays, the pickup/dropout ratio should be checked against the manufacturer’s
reference value.

ELECTRICAL TESTS ON THERMAL RELAYS

Thermal relays are frequently used for motor overload protection and may be
designed to provide protective sensing for single phasing, stalling and unbalanced
loading. The P&B Golds relay is the most common electromechanical type, and the
testing information given later reflects this. However, electronic relays, often with
some data processing capability, are now available in production quantities, and
these are designed to replicate the thermal characteristic of the motor in a similar
manner. The following tests are usually required as a minimum.

1. Check that each heater element operates in accordance with the calibration
curve, from cold, by applying a multiple of the rated current through the ele-
ments, having connected them all in series.
2. The time to indicate a given percentage (say 115%) of rated relay current whilst
the above multiple of the rated current is applied should be within the range
quoted in the manufacturer’s calibration data.
3. With the heater elements still connected in series check the hot operating
time by increasing the current from 100% running current to a multiple of