How are voltage and current measured?
On a series circuit the CCR fixes the current and the load sets the voltage. A clamp reads the current, a voltmeter reads the voltage, and the live loop is never opened.
In an AGL series circuit the current is the quantity the CCR sets: the same value flows through every transformer, and it steps with the brightness setting. The voltage is the result. It equals the current times the total circuit impedance, so it rises with each step and whenever load or cable is added. The two are therefore measured in different ways.
Current is read on the CCR display and checked with a true-RMS clamp meter closed round one cable, so the loop stays closed. Voltage is read across the output with a voltmeter in parallel, using an instrument and probes rated for the loop voltage, which on a large circuit can be several kilovolts. The clamp must also be rated for the circuit voltage; a general-purpose 600 V or 1,000 V clamp is not automatically suitable. The output of many CCRs, thyristor types especially, is not a clean sine wave, hence true-RMS. Never open a live loop to insert a meter: the CCR tries to hold the current, so the voltage rises across the break before any open-circuit protection acts, and that can be lethal. The loop current itself is hazardous, not only the voltage. Measure at an approved test point, as a competent person, under the site permit.
- Prepare. Get the permit, use a competent person, confirm the voltmeter, probes and clamp are rated for the loop voltage and in calibration, wear the specified PPE, and identify the approved test points.
- Select the step. Set the brightness step to be checked and let the output settle.
- Read the display. Note the step, output current, voltage and power shown by the CCR.
- Check current independently. Close a true-RMS clamp meter round one cable of the loop at the approved point.
- Check voltage. Where the site procedure allows it, connect a rated voltmeter or high-voltage probe in parallel across the output, never in series with the loop. Fit the leads with the CCR off and isolated, then energise; never reach into the live circuit. Isolate and lock out again before removing the leads.
- Compare and record. Compare with the value for the step and with earlier records; log the readings and investigate any difference.
What the readings suggest
Current matches the step
The CCR is regulating. Voltage then follows the load, so compare it with earlier records.
Current low, high or zero
Points to the CCR, an open loop, a fault or the instrument itself (for example a clamp around both conductors). Investigate under isolation, never by opening the live loop.
Voltage above records
The load may have increased, for example added fixtures or cable, or a high-resistance joint or poor connection.
Voltage below records
The load may have fallen, for example fixtures or transformers bypassed, or a cable section shorted.
Readings disagree
Check the instrument is true-RMS, in range and in calibration, and that both readings use the same test point and step.
Always record
Log the step, current, voltage, power, date and instrument, so changes can be trended over time.
Frequently Asked Questions
Q1How is current measured on a series AGL circuit?
Read the output current on the CCR display, and for an independent check use a true-RMS clamp meter closed round one cable of the loop at an approved test point, under the site permit. The loop stays closed, so no cable is cut or opened. Clamp one conductor only: clamping both conductors of a twin or multicore cable cancels the current and reads close to zero.
Q2How is voltage measured?
Voltage is measured across the CCR output, with the voltmeter connected in parallel and with an instrument and probes rated for the loop voltage. On a large circuit that can be several kilovolts, well beyond an ordinary multimeter, so use a high-voltage rated probe or divider as the CCR manufacturer and site procedure specify. The loop is also carrying several amperes, which is itself hazardous, so only a competent person measures it, at an approved test point, under the site permit.
Q3Why must a live loop never be opened to insert a meter?
The CCR tries to hold the current constant. If the loop is opened, the output voltage rises across the break before any open-circuit protection acts, which can be lethal and can damage the CCR. Never rely on that protection. Measure current with a clamp meter, and isolate, lock out, prove dead and earth the circuit under site procedures before any work that needs the loop opened.
Q4Why use a true-RMS instrument?
The CCR output is not a clean sine wave, particularly at the lower brightness steps of a thyristor-type CCR, so an average-responding meter can read wrongly. A true-RMS instrument gives a reading that matches the real heating value of the current and voltage. Check that the instrument is rated for the range and voltage, has adequate bandwidth and is in calibration.
Q5What should the readings be?
The current should match the value set for the selected brightness step. For a typical 6.6 A five-step loop that is about 2.8, 3.4, 4.1, 5.2 and 6.6 A; a three-step loop uses 4.8, 5.5 and 6.6 A. The voltage follows the load: it equals the current times the total circuit impedance, so it rises with each step and when load is added.
Q6Who can take these measurements?
A competent person trained for the hazard, working under the site permit and procedures, with instruments rated and in calibration for the voltage involved. Follow the CCR manufacturer's and the aerodrome's instructions for the test points.
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