Exide Technologies Section 93.20 - Manual (Page 18)

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SECTION V
SCR/SCRF SERIES BATTERY CHARGER CUSTOMER OPTIONS
The following pages describe customer options available in SCR/SCRF Series battery chargers. The text
describes the basic operational philosophy and theory of operation. Block diagrams (or schematics) for these
options are shown in Section VIII, in numerical order of the "EJ####" specification number.
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.
Descriptions:
a. POSITIVE / NEGATIVE GROUND INDICATOR LIGHTS (EJ0088)
The simplest 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. POSITIVE / NEGATIVE GROUND INDICATOR LIGHTS W/TEST SWITCH (EJ0089)
This is a variation of “EJ0088” above, so that the two lamps are not continuously connected in the
circuit. A double-pole, double-throw momentary center off switch (SW12) is utilized. Activating the SW12
down will connect the two lamps in series across the battery, with the center attached to chassis ground. This
performs as the two-lamp system (EJ0088) as described above. Activating the SW12 up serves to test the
lamps.
c. GROUND DETECTION SWITCH FOR DC VOLTMETER INDICATION (EJ0094)
This method uses the existing panel DC voltmeter (M2) and two double-throw switches (SW8/SW9)
to utilize the voltmeter in both functions. SW8 establishes the meter function (output voltage or ground
detection) and SW9 switches the voltmeter between (+) or (-) terminals 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 (EJ0086) - W/OPTIONAL INDICATOR LIGHTS
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 (SW3) must be activated to open the relays returning them to the “ready”
condition. Schematic EJ0086 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.