Ground Detection Circuits; Ac Power Failure Alarm Relay - HindlePower SCR series Operating Instructions Manual

1. GROUND DETECTION CIRCUITS

Purpose: The purpose of a ground detector circuit is to indicate to the user when either the positive (+)
or negative (—) output terminal of the battery charger (or its connected load) is grounded. This is
important for many applications where a full floating system is required for operation or safety reasons.
There are several methods of indicating a grounded output terminal. These will be outlined below with an
explanation of how each works.
Description:
a. TWO LAMP SYSTEM (EJ0088-XX): The most simple form of a ground detection system is the
two lamp method. This utilizes two lamps of voltage equal to the battery charger output voltage connected
from the positive terminal to ground and from negative terminal to ground. Under normal conditions, each
bulb will have half-voltage applied and glow dimly at quarter-brilliance. Upon a ground, the lamp
indicating the grounded terminal will glow at full brilliance. The other lamp will go out.
b. TWO LAMP SYSTEM WITH TEST SWITCH (EJ0089-XX): This is a variation of "a.", above, so
that the two lamps are not continuously connected in the circuit. A double-pole, double-throw, momentary
center off switch is utilized. Activating the switch in one direction will connect the two lamps in series
across the battery with the center attached to chassis ground and perform as the two lamp system, EJ0088,
as described above. Activating the switch in the other direction serves to test the lamps.
c. SWITCHABLE DC VOLTMETER METHOD (EJ0094-XX): This method uses the existing panel
DC voltmeter and two double-throw switches to utilize the voltmeter in both functions. Switch SW8
establishes the meter function (output voltage or ground detection) and SW9 switches the voltmeter
between (+) or (—) terminal to chassis ground reversing the meter polarity to maintain up-scale
indications. The voltmeter, used in the ground detection mode, will indicate the DC voltage potential
between the corresponding output terminal and chassis ground. For cases of partial grounds, the voltmeter
will indicate the difference of the terminal DC voltage and the voltage of the partial ground.
d. GROUND DETECTION ALARM RELAYS WITH OPTIONAL LIGHTS (EJ0086-XX): This
method utilizes two DC relays which monitor the (+) and (—) terminal voltage to chassis ground and will
energize if the opposite polarity terminal is grounded. To insure that the relays will not energize on half-
voltage, when there is no ground present, series resistances are used to reduce the relay pull-in sensitivity.
However, once pulled in by a ground condition, the relays will not drop out when the ground is removed
because the drop out voltage of a DC relay is always less than the pull-in voltage. As a result, a
momentary "RESET" switch must be activated to open the relays returning them to the "ready" condition.
The EJ0086 schematic shows the circuit diagram of this method. The panel lamps shown are a user option
that is available for front panel indication. The relay coil voltages, and series resistances will depend on
the output voltage of the battery charger.
2. AC POWER FAILURE ALARM RELAY (EJ0085-XX) WITH OPTIONAL LIGHT
Purpose: The purpose of this alarm is to notify the user that the AC input power to the battery charger
has been interrupted. A front panel lamp is optionally available for local indication. Relay contacts are
provided for remote monitoring.
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