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portion of the wing root. Aircraft serial numbers C0149 and C0151 onwards have this receptacle
located on the fuselage in front of the left-hand wing root. See Figure 1.
2. Description
The two main components of the external power source are the EPU receptacle and the relay.
The EPU receptacle and related circuits provide for the connection of an external power source for
various ground operations, e.g. maintenance, battery charging, starting.
The external power relay is mounted on the forward face of the cockpit front bulkhead. Heavy duty
positive and negative cables are connected to the relay terminal. Another heavy duty cable is
connected to the solenoid of the external power relay via the diode.
3. Operation
The external power system provides protection in the event that the external power source is
connected in reverse polarity. A switch in the cockpit to the left of the light switches allows the EPU
relay to close once the external power source is connected and power is available. A light in the
cockpit indicates that the power is available at the receptacle or that the EPU relay has remained
closed when it was disconnected.
On aircraft C0001 through C0148 and C0150 with an EPU installed, a relay bypass circuit is
provided to enable the battery relay to be closed if the battery has been discharged. When the
battery does not have enough power to close the relay by itself, the relay bypass circuit enables the
battery relay to close. Depending on the state of battery discharge, the battery relay may take
several minutes to close. This circuit is not installed on aircraft C0149 and C0151 onwards. Refer
to Figure 2, simplified schematic diagram of the EPU.
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Figure 1 – EPU Receptacle Locations
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Figure 2 – Simplified Schematic Diagram
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TROUBLE-SHOOTING
1. General
The table below lists the defects with the external power unit.
Complaint Possible Cause Remedy
No external power supply in the
system.
External power source is not
connected.
External power relay is
defective.
Make sure the external power
source is securely plugged
into the receptacle.
Test the external power relay.
Replace the external power
relay if defective.
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MAINTENANCE PRACTICES
1. General
This section shows how to remove/install the external power relay. Refer to the manufacturer’s
manuals for more information on the external power relay.
2. Electrical Safety
The DA20-C1 has a low-voltage DC electrical system. When correctly maintained it is safe to work
on. But the battery can supply heavy current through low-resistance circuits (for example, if you
ground the positive output with a wrench by accident).
Always follow the usual safety practices for working on electrical equipment. Allow only qualified
persons to maintain the electrical system.
CAUTION: DISCONNECT THE BATTERY BEFORE DOING MAINTENANCE ON
THE ELECTRICAL SYSTEM. MAKE SURE YOU DISCONNECT THE
NEGATIVE LEAD FIRST.
CAUTION: AFTER DOING ELECTRICAL MAINTENANCE ALWAYS DO A
CONFIDENCE TEST OF THE SYSTEM WITH A 14 VOLT POWER
SUPPLY THAT HAS OVER-CURRENT PROTECTION. DO THIS
BEFORE CONNECTING THE BATTERY
CAUTION: USE ONLY DA20-C1 SPARE PARTS APPROVED BY THE
MANUFACTURER.
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3. Remove/Install the External Power Relay
A. Remove the External Power Relay
Detail Steps/Work Items Key Items
(1) Disconnect the battery for maintenance. Refer to Chapter 24-31.
(2) Disconnect the cable from the external
power relay:
− Remove the nuts and washers from the
terminal of the external power relay.
− Move the cable clear of the way.
(3) Remove the bolt, washer, and nut that
attaches relay to the firewall.
(4) Remove the external power relay.
B. Install the External Power Relay
Detail Steps/Work Items Key Items
(1) Move the external power relay into the
　
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