Video: RMO-A

Video transcript

What the RMO-A measures, how it is connected to a circuit breaker, and how a test is run, from the first clamp to the stored result. A breaker contact that has gained a few micro-ohms looks normal. At load current those micro-ohms become heat, and heat speeds up the wear. To see it in time, a high direct current is passed through the contact and the voltage drop across it is measured. Six models, from 100 to 600 amps of ripple-free direct current, with a resolution of 0.1 micro-ohm, in 7 to 8 kilograms.

The front panel holds everything used in a test: the display and keys, the status lights, START and STOP, the USB port, and the two current sockets with the voltage sense sockets between them. At the back, the mains inlet and the switch, the fan, and the one that matters most: the earth stud. It is the first connection made and the last one removed. It ships with the PC software and the USB, mains and earth cables. The test leads come in sets: two current cables with battery clamps and two sense cables with alligator clips,

5 metres as standard, sized to the model. The order is fixed. The breaker is isolated and earthed. The RMO-A is earthed and switched off. The leads go into the instrument first, then onto the breaker. Coming off, the reverse, with the earth last of all. The connection is four-wire Kelvin. Current leads on the outside, sense leads inside them, as close to the contact as possible, so the cables and clamps stay out of the result.

Red with red, black with black. On a live tank breaker the leads go onto the terminals at each end of the interrupter. On a dead tank breaker, onto the two bushings. In both, the sense clamps sit inside the current clamps. Hold SET for 3 seconds to reach the settings: the language, the date and time that stamp every result, and Rmax, a pass or fail limit that lights the red LED.

A single test needs two values: the current and the memory position. ENTER, then START. The cable connection is checked, the current ramps up, is held for one second while the voltage drop is measured, and ramps back to zero. The ramps keep magnetic transients out of the reading. Dead tank breakers have their own mode. There the test current also flows through the bushing current transformers, and while they magnetise, the reading drifts.

DTRtest measures every second, stores the result only once it detects saturation, and then stops by itself. In CONTIN mode the current is held for a set time, with the resistance shown live. Up to 100 amps there is no limit. At 600 amps it holds 20 seconds, then rests 6 times as long, and the instrument enforces it with a countdown. The result screen shows the resistance, the voltage drop, the current reached and the memory position.

ENTER and START repeat the test one position further on. If the current was not reached, the object or the cables are too resistive. An error lights the red LED, sounds the buzzer and names the problem. Almost every message points to a cable: a sense lead off, a current lead open, or the polarity reversed. If the results look wrong, check the instrument first. Two test shunts in series, 100 micro-ohms and one milliohm, connected just like a breaker.

If it reaches the set current and reads within tolerance, the fault is in the connection. Over USB, the PC software controls the whole instrument. In CONTIN mode it logs every reading at the chosen sampling time, where the instrument keeps only the last one, and builds the table, the graph and the report. It measures any low resistance that has to carry high current: breakers, disconnectors, busbar joints, cable splices, welded joints and fuses, from the factory to maintenance.

Circuit breaker testing, from Amperis.