Validation of specialist current measurement devices wins over new customers

A wide range of industries face a challenge of accurately measuring extremely high alternating currents (AC), often in harsh or space-constrained environments, without compromising safety, accuracy, or system performance.  

This problem appears in such diverse fields as nuclear fusion research, electric arc furnaces, high-current switchgear, fuse testing and sudden short-circuit testing, as well as some low-current applications including measuring signals and faults on railway tracks and assessing whether variable speed drives interfere with railway signalling processes. In each case, engineers need a measurement solution that can handle a very wide range of current magnitudes, rapid changes in current, and electrically noisy conditions, all while remaining compact and easy to install. 

Rocoil has such a solution – a device based on Rogowski coils, which its 12-person team in Yorkshire sells around the world. A Rogowski coil is a device containing an air-cored coil and electronic integrator that measures alternating currents (AC) and high current pulses. While the Rogowski coil is not as well-known as other transducers, they can measure a much wider range of currents – from less than an amp to millions of amps – and are very compact.  

The Challenge 

Because these devices are non-standard measuring instruments, questions are often asked about whether calibrations are accurate. Rocoil calibrates the devices in-house at low currents (hundreds of amps) to show their accuracy, but it has been difficult to prove accuracy at high currents because few institutions offer high current testing.  

Until now, Rocoil relied on the theoretical assumption that the coils show measurement linearity – i.e., that if the coil measured accurately at low currents, it would scale proportionally and remain accurate at high currents, and so does not need specific calibration at high currents. But this wasn’t proven.  

Many of Rocoil’s larger customers have been asking for a form of certification that the calibrations are accurate at high currents before deploying them at scale, and the company has missed out on business opportunities due to lack of such proof. 

To prove measurement accuracy, Rocoil approached NPL through the Measurement for Business (M4B) programme. 

The Solution

At NPL, the coil was tested to show accuracy at both high and low currents. NPL precisely generated a range of currents up to 10,000 amps and measured them with both the Rogowski coil and a highly accurate in-house current measurement device.

The objective was to demonstrate that the Rogowski coil produced results consistent with those of the in-house reference device, confirming measurement comparability under similar test conditions. In doing so, it could be shown that a Rogowski coil calibrated at low current retains linearity when operated at high current.

The results demonstrated close agreement between the coil readings and those from NPL reference devices, confirming linear scaling and accurate performance of the Rogowski coil at elevated currents, and thereby validating Rocoil’s claim of linearity for the coil under test.

The Impact 

The M4B project has provided reassurance for both Rocoil and their customers that the calibrations are accurate across high and low current applications, and the device is backed by validation from NPL – which is available on Rocoil’s website and can be directly sent to customers on demand. 

This validation has already brought in new customers and led to new customers and orders being placed, and Rocoil expects it will provide a 10% increase in orders. This includes new customers, existing customers, and potential customers that had already contacted Rocoil but were hesitant to purchase the coils without the high current validation. 

The certification has expanded Rocoil’s business within their existing markets and it but could open the door to new industries with strict certification and validation requirements.