Video: PDRM-1A

Video transcript

How the PDRM-1A measures low resistances with one amp, how its 4 leads go on, and where a one amp instrument is the right choice. An earthing grid has 100 of connections, and each one can corrode or loosen underground. Measured one by one with 4 wires, and compared with a sound one, the weak joints stand out before a fault current finds them. It measures from one micro-ohm to 200 ohms, with one micro-ohm resolution over 4 and 3 quarter digits, and an accuracy of 0.1% of reading.

It chooses the range by itself, in 860 grams. Six scales, chosen automatically. The two lowest, 2 and 20 milliohms, use the full amp. Then 100 milliamps up to two ohms, and 5 milliamps for 2200 ohms. On top, 5 sockets: yellow and green for the current, red and black for the voltage, blue for an external trigger. On the front, power, GO for a new reading, MODE and the backlight, with a display that can be read in the dark.

Two leads inject the current at the ends of the joint, and two more read the voltage drop across the section under test. The resistance of the leads and contacts stays outside the reading, and the filtered inputs keep it stable near live busbars. Each lead goes into the socket of its own colour. Yellow and green clip onto the two ends of the object: they carry the pulsed current. Red and black go between them.

The reading then starts by itself. With dual-hand probes, each probe carries a current tip and a voltage tip. All 4 spring tips must bite, and the black dots on the probes must face each other: that keeps the voltage tips on the inside. Hold still until the reading is done. Kelvin clamps carry both leads in one jaw. Clamp them firmly and give them a slight twist to break through the oxide. The resistance is measured between the innermost jaw surfaces.

MODE steps through standard, relative, relative in percent, motor mode for small windings, and OK or X against a reference. Held down, it opens the settings: mains frequency, language and contrast. Relative mode is the method for a series of earth connections. Measure one known to be good, press MODE, and every new joint shows its deviation from it. The deviation, not the absolute value, points at the joints to open.

Exothermic welds in substation grids are checked without cutting them, with a probe of 4 fixed spring tips. Clean the weld, press until all 4 engage, and compare each one with a sound weld. It comes with Kelvin clamps, the batteries, the manual and a calibration certificate. As options: dual-hand probes with spring tips, Kelvin clamps of 35 and 19 millimetres, the weld probe, a cable elbow accessory, a reference resistance and a pouch or a hard case. Six rechargeable AA cells give more than 4000 readings

at one amp and more than 10000 at the lower currents. The backlight goes off after 30 seconds and the instrument after two minutes. BAD CONNECTION means a contact to check. Next to the 10 amp PDRM-10A, the PDRM-1A works with a 10th of the current and one micro-ohm resolution. That is enough for earthing grids, welds and connections, and the low current suits small contact points. Four rules.

Measure only on de-energised equipment, and treat every terminal as live. The instrument and its adapters are not insulating. No flammable gas or explosive areas. And only the approved batteries and charger. It is used to check earth connections and power busbars against a reference, welded joints and mechanical connections, circuit breaker contacts and power fuses, castings and electrodes, and exothermic welds. Low resistance measurement, from Amperis.