Measuring circuits

Seven procedures. Guided first attempts with a meter are in GEN-A Hands-on.

Before every one: predict the reading. GEN-A Concepts §2 — a measurement without a prediction is a number.

Procedure 0 — Before the probes touch anything

1. Know what circuit you are on, from the wiring diagram.
2. Know whether it is live, and make it dead if it should be.
3. Set the meter to the right function and range before connecting, not after.
4. Check the leads — in the right sockets for the function, insulation intact.
5. Know what is on the other end. R5 in GEN-K Concepts §4: an ohmmeter pushes current, and some circuits must never receive any.

Check — say what you expect to read. If you have no expectation, you are not ready to measure.

Procedure 1 — Continuity test (S1)

Circuit de-energized. A continuity tester supplies its own current; a live circuit destroys the meter.

1. Isolate the conductor at both ends.
2. Set the meter to continuity or the lowest resistance range.
3. Zero it by touching the probes together, and note the lead resistance.
4. Measure end to end. Subtract the lead resistance.

Check — touch the probes together first, every time. Test leads have resistance, and on a short cable run that resistance is a large fraction of what you are measuring.

Failure — treating continuity as proof of a good conductor. It proves connection, not quality (GEN-A Concepts §3).

Procedure 2 — Measure voltage (S2)

Meter in parallel, across the thing you are measuring. Circuit live.

1. Set to DC or AC volts as appropriate, on a range above the expected value.
2. Connect across the load or between the point and a known ground.
3. Read.

Check — measure a known good point first. Bus voltage at the breaker confirms the meter, the leads and your reference before you go hunting.

Failure — measuring AC on a DC range or the reverse. Note whether the system is AC or DC before you set the meter; on aircraft with both, that is a real question.

Procedure 3 — Measure current (S3)

Meter in series, in the circuit path. This is the measurement that damages meters.

1. De-energize.
2. Break the circuit and insert the meter in series.
3. Use the current sockets, on a range above the expected value.
4. Energize, read, de-energize before disconnecting.

Check — a meter set to current has almost no resistance. Connecting it across a supply, as you would for voltage, is a short circuit through the meter. Before energizing, confirm the meter is in series with a load and not across the bus.

Failure — leaving the leads in the current sockets and then measuring voltage. This is the commonest way meters are destroyed and it is entirely preventable by returning the leads to the voltage sockets after every current measurement.

A clamp meter measures current without breaking the circuit and is preferable where available and where the current is DC-capable.

Procedure 4 — Measure resistance (S4)

Circuit de-energized, and the component isolated from the rest of the circuit.

1. De-energize and disconnect at least one end of the component. Parallel paths give a reading that is not the component's resistance.
2. Zero the leads.
3. Measure.

Check — compare against the expected value. A resistor's color code (K20), a component's specification, or a like component. A reading with nothing to compare it to tells you almost nothing.

Failure — measuring in-circuit and reading the parallel combination of everything connected.

Procedure 5 — Voltage drop test (S11) — the one that finds faults

Circuit live and loaded. That is the point.

1. Energize the circuit and turn the load on. Current must be flowing.
2. Set the meter to a low DC volts range.
3. Place the probes across the element you suspect — the two sides of a connector, the two ends of a wire run, the case and the airframe for a ground.
4. Read. A good connection drops close to zero.
5. Work along the circuit, element by element, until you find where the voltage is being lost.

Check — the drops around the circuit must sum to the supply voltage. If they do not, you have missed an element — and the missing volts are in whatever you have not measured yet.

Failure — testing with the load off. With no current there is no drop, every joint reads zero, and the circuit appears perfect (GEN-A Concepts §3).

Procedure 6 — Test for shorts and opens (S10, S12)

Open circuit — de-energized, disconnect both ends, measure continuity. Infinite resistance where you expect near zero is an open. Voltage drop locates it faster if the circuit can be energized: the whole supply voltage appears across the break.

Short to ground — de-energized, disconnect the conductor at both ends, measure between the conductor and airframe. Near-zero resistance where you expect infinite is a short.

Short between conductors — same method, between the two conductors, both disconnected.

Check — an open reads infinite where you expect zero; a short reads zero where you expect infinite. State which fault you are testing for before you measure, so you know which reading is the bad one.

Failure — testing for a short with one end still connected, so you are reading through the rest of the aircraft.

Procedure 7 — Service a battery (S14)

Establish the chemistry first (GEN-A Concepts §5). Lead-acid and nickel-cadmium need different PPE, different neutralizers and different servicing equipment.

1. PPE: face and eye protection, gloves, apron. Ventilation.
2. Remove metal from your hands and wrists.
3. Disconnect the earth lead first.
4. Inspect for cracks, corrosion, distortion and vent condition.
5. Check electrolyte level and specific gravity per the manufacturer, using equipment dedicated to that chemistry.
6. Add the specified water only, to the specified level, at the specified time relative to charging.
7. Clean terminals, reassemble, connect earth lead last.
8. Record the servicing.

Check — the correct neutralizer must be present before you start, and you must be able to say which it is. If you cannot name it, you have not established the chemistry.

Failure — a spanner across the terminals. Aircraft batteries deliver hundreds of amps into a short; the tool welds and sprays molten metal.

Procedure 8 — Read a diode or a logic gate on a diagram (S7)

Source: FAA-H-8083-30B Ch. 12, Diode Identification, Logic Polarity and Basic Logic Circuits.

A diode

1. Find the cathode. On the symbol it is the bar; on the part it is marked by a band, a dot, a colored band, the letter "k" or an odd shape.
2. The diode conducts when its anode is positive relative to its cathode (forward bias). It blocks current the other way.
3. Say which way current can flow through it in the circuit you are reading, and what that prevents — usually current feeding back from one bus or load into another.

A logic gate

1. Inputs are on the left, the output on the right.

2. Identify the gate from its symbol. A small circle on an output means the output is inverted: an inverter (NOT), NAND or NOR.

3. Apply the rule:

   Gate	Output is high when
   AND	all inputs are high
   OR	any input is high
   NOT (inverter)	the input is low
   NAND	any input is low — the opposite of AND
   NOR	all inputs are low — the opposite of OR

4. Check the logic polarity on the manufacturer's diagram. It states which voltage means 1, and in negative logic the more negative level is the 1.

Check — say the condition in words. The handbook's two examples: autopilot engage is an AND (vertical gyro valid and directional gyro valid and the rest); a "DOOR UNSAFE" light is an OR (cabin door or baggage door not secure).

Failure — reading a NAND as an AND because the small circle was missed.

The checks, collected

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