L17 Avtron Load Banks — Lesson Companion

This guide is a companion to the Lesson 17 — Avtron Load Banks video. Use it to follow along during the lesson, to reinforce the major components and their functions, and to locate troubleshooting information quickly afterward.

Watch for these callouts as you read:

This lesson covers three Avtron load bank configurations used at FAA facilities:

1. Radiator Mounted Manual Load Bank — non-shed, operates on commercial power
2. Radiator Mounted Automatic Load Bank — load shed, operates on emergency power
3. Remote Automatic Load Bank — outdoor automatic load shed with blower system

Each type builds on the previous. The manual load bank establishes the core circuit logic. The automatic load bank adds load shed control. The remote load bank adds airflow proving and environmental protection.

Required materials: Schematic book, ASCO 962 operator manual, ASCO 434 operator manual

Lesson objective: Given an Avtron load bank schematic, identify components, describe their functions, and describe system operation in accordance with applicable documentation.

Load banks allow testing of emergency generators under simulated load with minimal impact to facility operations.

Periodic load testing is used to verify that engine generators can sustain rated load and support facility operations during an emergency power event.

What resistive load banks do:

• Convert electrical energy to heat through resistive elements
• Apply a controlled load directly to the generator output
• Allow the generator to be tested at operating load without loading the facility
• Verify that the standby power system is ready to support facility operations

Typical load shed operating target: 50–80% of rated generator capacity

Operating below 50% risks wet-stacking in diesel engines. Operating above 80% risks overload. All three load bank types are designed to keep the generator within this range during testing.

Note: Specific FAA load-testing requirements and acceptable test loading methods are defined by current maintenance guidance and may vary by equipment type and testing objective.

What It Is

The manual load bank is a non-shed load bank. "Non-shed" means:

• It has no automatic load shedding capability
• All load steps are applied manually by the technician
• It is disabled by design during emergency power operation

It operates only while the ATS is on commercial power. When the ATS transfers to emergency, control power is removed and the load bank shuts down automatically.

Control Panel Components

Load Bank Components

How Control Power Flows — Manual Load Bank

Control power must pass through every item in this path before K99 can energize:

120V AC source
    → TB1-1
    → F1 (10A fuse)
    → S23 (emergency stop, normally closed)
    → TB1-8 (exits control panel)
    → TS12 / TS13 (ATS feature 14A — must be on commercial)
    → TB1-7 (returns to control panel)
    → S20 (control power switch, closed)
    → TB1-6 (exits to load bank)
    → TB11-6 (enters load bank)
    → S60 contact 1-to-3 (normal, not over temp)
    → TB11-4 (exits load bank)
    → TB1-4 (returns to control panel)
    → K99 coil → K99 ENERGIZES

When K99 energizes: K99-4 and K99-7 close → DS20 illuminates → load bank ready.

Applying Load — Procedure

1. Close S20 (control power switch) → DS20 illuminates, K99 energizes
2. Close S21 (master load switch)
3. Apply load steps one at a time
4. Target: 50–80% of rated generator load
5. For the 30 kW schematic: apply any two steps (20 kW) to reach target range

Valid two-step combinations: S1+S2 | S1+S3 | S2+S3

Shutdown Conditions

The manual load bank stops operating when any of the following occur:

Over Temperature Protection — S60

S60 is the most important safety device in the load bank.

When S60 trips:

• K99 de-energizes
• DS21 (over temp LED) illuminates in the control panel
• Load bank is de-energized and loses all load
• Engine continues running
• Technician loses load bank control until S60 resets

Troubleshooting — Manual Load Bank

What It Is

The automatic load bank is a load shed system. It:

• Operates only while the ATS is on emergency power
• Automatically adds load bank steps when facility load decreases
• Automatically removes load bank steps when facility load increases
• Maintains the generator in the 50–80% load range without technician intervention

FAA application: Used on Kohler generator sets that do not have an Electronic Control Module (ECM), including many first-generation Kohlers.

Feature 14B and ATS Contacts

Control Panel Changes

The automatic load bank shares most control panel components with the manual version. Key changes:

K100 and K101 — The FAA Modification

When the ATS transfers to emergency, TS10/TS11 close and energize both K100 and K101.

K100:

• Opens normally closed contacts in series with S1, S2, and S3
• Prevents the technician from manually adding load steps while the generator is carrying actual emergency facility load
• Protects against manual engine overload

K101:

• Opens normally closed contacts in series with S4 — disables manual step S4
• Closes normally open contacts — routes 120V AC control power through TB1-9 and conduit to TB11-9, enabling the load shed assembly at connector J1

CT1 and Load Monitoring

CT1 is the current transformer that provides feedback to the automatic load shed assembly.

• CT1 monitors the generator output current, which reflects the facility load being carried by the generator
• CT1 converts the high current levels to a proportional lower-level signal
• This signal feeds the load shed assembly at connector J1, terminals 1 and 2
• The load shed assembly uses the CT1 signal to determine when to add or remove load steps

Automatic Operation Sequence

1. 120V AC enters panel at TB1-1 → F1 → S23 → S20 (Auto position) → DS20 → TB1-6 → TB11-6 → S60 → TB11-4 → TB1-4 → K99 energizes
2. ATS transfers to emergency → TS10/TS11 close → K100 and K101 energize
3. K100 opens S1/S2/S3 manual paths — manual step control disabled
4. K101 N/O contacts close → 120V AC to TB1-9 → TB11-9 → load shed assembly connector J1
5. K99-4 and K99-7 close → control voltage to auto on LED and to load shed assembly
6. Load shed assembly begins automatic step control based on CT1 feedback

What It Is

Detail A-A is the load shed assembly control circuit. It is shown as an inset on both the radiator-mounted automatic and remote automatic load bank schematics.

Purpose: Keep the generator loaded at 50–80% of rated output by continuously monitoring the facility load carried by the generator and automatically adding or removing load bank steps.

Inputs — Connector J1

Outputs — Connector J2

The load shed assembly sends load step commands out through J2:

Load Step Control Relays — K41, K42, K43

K41, K42, and K43 are the load shed controller's output relays. Each corresponds to one load step.

Important: these relays use inverse logic.

K41, K42, K43 relay terminals visible on Detail A-A:

Timing Relays — K40, K31, K32, K33

K40, K31, K32, and K33 are the sequencing relays that gate each load step in a timed chain. They prevent all load steps from being applied simultaneously.

Total minimum time from power application to all steps available:
5 + 3 + 3 + 3 = 14 seconds

How Each K3x Relay Works

Each K3x relay has three contact functions:

For a load step to apply, two conditions must both be true:

1. The K3x sequencing relay must be energized (contact 18 closed — gating K4x)
2. The K4x relay must be de-energized (E3 contacts in closed position)

Neither condition alone is sufficient.

Load Shed Operating Logic

Example: 100 kW generator. Target = 50–80 kW.

Facility load is in range (50–80 kW):

• CT1 confirms optimal loading
• Load shed controller energizes K41, K42, K43
• K41/K42/K43 E3 contacts open → all load bank steps removed
• Load bank on standby

Facility load falls below 50 kW:

• CT1 signals low load
• K41 de-energizes → K41 E3 contacts close → K31 path gates → J2-2 signals K1 → Step 1 applied
• Still below 50 kW: K42 de-energizes → K32 → J2-4 → K2 → Step 2 applied
• Still below 50 kW: K43 de-energizes → K33 → J2-6 → K3 → Step 3 applied
• Continues until CT1 confirms 50–80% range

Facility load rises above 80 kW:

• K43 energizes → K33 de-energizes → Step 3 removed
• Still above 80 kW: K42 energizes → K32 de-energizes → Step 2 removed
• Still above 80 kW: K41 energizes → K31 de-energizes → Step 1 removed
• Continues until generator returns to range or all steps removed

Load steps are applied in order: 1 → 2 → 3
Load steps are removed in order: 3 → 2 → 1

What Is the Same

The remote automatic load bank uses the same core logic as the radiator-mounted automatic load bank:

• 120V AC control power
• K99 gatekeeper relay (required)
• Feature 14B / TS10/TS11 for automatic enable
• K100 and K101 FAA modification relays
• Load shed assembly — Detail A-A circuit
• CT1 for load monitoring
• K40/K31/K32/K33 timing sequence
• K41/K42/K43 load step control
• S60 over temperature protection

What Is Different

The remote load bank is located outside, away from the engine-generator set. Without access to radiator airflow, the unit requires a blower motor to move air through the resistive elements.

Before any load can be applied, the blower must prove it has reached 900 CFM. K99 will not energize until airflow is proven.

New components unique to the remote load bank:

Blower Circuit

The blower operates on 208V AC supplied through TB12.

Normal Startup — Airflow Proving Sequence

At initial power-up, two things happen simultaneously:

Path 1 — Air fail indication:
S60 (normal, 1→3) → S61 (normal, below 900 CFM) → exits at TB11-5 → TB1-5 → DS21 Air Fail illuminates

Path 2 — Blower start:
K21 N/C contacts → K22 coil energizes → K22 contacts close → 208V AC through F2/F3/F4 → through K21 motor contacts → blower starts → DS22 Blower Power illuminates

At startup, both DS21 and DS22 are lit simultaneously. This is normal.

Cold Weather Devices

K21 Overload — Cascade Shutdown

If the blower motor draws excessive current for a prolonged period:

K21 bimetal strip heats → K21 contacts open
    → K22 coil de-energizes → K22 contacts open
    → 208V AC removed from blower motor
    → Blower decelerates below 900 CFM
    → S61 switches back to normal position
    → K99 coil loses power → K99 de-energizes
    → Load shed assembly de-powered
    → Load bank inoperable (auto and manual)

Result: DS21 (Air Fail) ON | DS22 (Blower Power) OFF

Engine continues running. Only the load bank is disabled.

Outdoor Maintenance Awareness

Remote load banks are typically located outdoors. Before opening the enclosure:

• Visually inspect the exterior for signs of nesting (mud tubes, insect activity, droppings)
• Use a flashlight to inspect the interior before inserting hands
• Documented enclosure residents: snakes, mice, rabbits, squirrels, bees, wasps, spiders

Pitot tube blockage:
The pitot tube provides the airflow velocity signal to S61. Mud daubers (a species of wasp) are documented to nest inside the pitot tube. A blocked pitot tube prevents S61 from sensing adequate airflow — even when the blower is running at full speed.

Result of blocked pitot tube: DS21 (Air Fail) remains lit, K99 does not energize, load bank inoperable.

Load Bank Types and ATS Conditions

• Manual = commercial power (feature 14A, TS12/TS13)
• Automatic = emergency power (feature 14B, TS10/TS11)
• Remote = emergency power + airflow proven (feature 14B + S61)

K99 — The Gatekeeper

• Required for all three load bank types
• Required in both manual and automatic modes
• Cannot be bypassed
• Failure = load bank inoperable

S60 Temperatures

• Trip: 350°F
• Reset: 310°F
• Contact change: 1→3 (normal) to 1→2 (over temp)

Timing Relay Sequence

• K40: 5 seconds
• K31: 3 seconds
• K32: 3 seconds
• K33: 3 seconds
• Total minimum: 14 seconds

K41/K42/K43 Logic

• Energized = load step removed
• De-energized = load step applied

Seven Manual Shutdown Conditions

1. Load step switches opened
2. Master load switch (S21) opened
3. Emergency stop (S23) opened
4. Control power switch (S20) opened
5. Loss of commercial power
6. ATS transfers to emergency
7. Over temperature (S60 trips)

S61 Threshold

• 900 CFM — remote load bank only
• Below 900 CFM: S61 routes to DS21 air fail
• At 900 CFM: S61 switches to K99 coil path

S62 Threshold

• Closes below 50°F — remote load bank only
• Energizes HR1 heater

K100 / K101 Functions

• K100: Energized by TS10/TS11; disables manual steps S1, S2, S3
• K101: Energized by TS10/TS11; disables manual step S4; closes N/O contacts to power load shed assembly

Optimal Generator Load Range

• 50–80% of rated capacity
• Example: 100 kW engine → target 50–80 kW

CT1 Purpose

• Monitors facility load being carried by the generator
• Provides feedback signal to load shed assembly at J1-1 and J1-2
• Enables automatic load step add/remove decisions

Three load bank types — different sources, same goal:

K99 is the common thread. Every load bank type, every operating mode, requires K99. If K99 cannot energize, the load bank cannot operate. Understanding what K99 needs — and what can prevent it from energizing — is the foundation of load bank troubleshooting.

The automatic load shed assembly (Detail A-A) adds two relay groups:

• K40/K31/K32/K33 — a 14-second timed sequencing chain that gates step availability
• K41/K42/K43 — the controller's output relays that apply or remove steps based on CT1 feedback

Both relay groups must cooperate for a load step to apply or be removed.

The remote load bank adds one requirement: the blower must prove 900 CFM airflow before K99 can energize. Every additional component in the remote load bank (S61, K21, K22, DS21, DS22, pitot tube) serves this one function.

Periodic load testing and post-maintenance testing of applicable engine generators are standard practice in aviation facility maintenance. Proper maintenance and troubleshooting skills directly support mission-critical facility uptime.

L17 Avtron Load Banks — Lesson Companion
Course: 40167002 Kohler Engine Generator Power Systems
References: Avtron load bank schematics D26193, D27496, D07714; ASCO 962; ASCO 434