Once I showed a meter to Sanjay Saab in our AECo (Ahmedabad Electricity Co Ltd – presently Torrent Power) meter laboratory, which was in a cast iron box and had three wires with soldered lugs projecting out on both sides. It was a two-element three-phase three-wire 400-volt 200-amp meter, with a built-in bar-type current transformer (CT), with a CT-secondary, directly connected to the meter. I do not have a photo of this meter, but to get a feel of it, I made a Photoshop-constructed replica.
Sanjay Saab loved this idea. Immediately, he started narrating the advantages of such a 200-ampere meter:
On another visit, he asked me to show him the meter again. I am sure that he had been thinking of having a direct 200-ampere meter. We discussed the possible methods of avoiding the cable and lugs and about making through holes, to pass an insulated conductor through the meter box and the CT. But since the meter needs potential, how could it be possible to tap the potential in the meter box on an insulated conductor after its insertion through the CT?
On a subsequent visit, he came up with the idea of a piercing clamp, which would avoid peeling off the conductor insulation and make the associated potential available to the meter.
After a well-thought-out product development, which went through many design changes and overcame various issues, a meter (Prodigy) was eventually developed with four CTs. By this time, every meter engineer was convinced that a three-phase three-wire (two-element) meter has many disadvantages.
Prodigy was designed for outdoor installation without any meter box or rain sheds. It was thoroughly tested under a water shower in the laboratory and passed all the tests.
For on-site testing, a few samples were installed in the field, and after the first rain showers, all of them were returned as a lump of plastic and metal. We learned that this was because pollution, together with moisture, had created problems.
Again, the design underwent a change to create creepage (breaking the water path). The final version was tested in a live condition under a polluted water shower until it passed the test. Polluted water was created in the laboratory by mixing water with sea salt and dry carbon dust collected whilst cleaning the silencers of vehicles from Pratapnagar Choraha (crossroads close to our head office).
In one state, the Power Minister, who was aware of the mischief played out with CTs and meter wiring, would say, “I want a meter without chuddy [ring CT].” – i.e. Prodigy.
Prodigy was developed as an outdoor meter with all the latest features for tamper detection, remote reading and communication. Many utilities have mounted Prodigy on a pole at the factory gate to avoid fiddling by consumers.
-G K Panchal