Metrosil Celebrates 85 Years of Production

Metrosil, the world’s leading brand of silicon carbide varistors, which protect critical infrastructure around the world, including the world’s biggest hydroelectric dams, is celebrating 85 years of production.

“Metrosil’s properties and composition are what makes it incredibly useful for critical fault protection. It’s certainly stood the test of time and has always been very ‘current’; not only will you find Metrosil in use today on some of the world’s largest hydropower projects and nuclear power stations – but you’ll find it on course to protecting the fusion power plant of tomorrow,” Michael Anderson, Managing Director – Speciality Products, M&I Materials.

Manufactured by M&I Materials at Trafford Park in Manchester, Metrosil varistors have come a long way since their launch by the Metropolitan-Vickers Electrical Co. Ltd. in 1937. Metropolitan-Vickers Electrical Co. Ltd was a British electrical engineering company and for most of the 20th century, their factory in Trafford Park, Manchester, was one of the biggest and most important heavy engineering facilities in Britain and the world. Back then, Metrosil was primarily used to protect electric power transmission and telecommunication systems from the damaging effects of lightning. Today, not only does Metrosil protect the world’s largest hydroelectric dams – it protects the world’s largest and most complex scientific instruments.

Developed by Dr. Ronald Sillars, who was developing electrical insulating materials using silicon carbide, Metrosil marked a significant departure from existing understandings of resistant materials. Ohm’s law saw current as directly proportional to voltage, however doubling the voltage applied to Metrosil increased the current twentyfold. With a non-ohmic resistance Metrosil had an obvious application in lightning protection, an application which was rapidly implemented. Gradually its uses were extended to telephone and radar equipment. During the Second World War, Metrosil was heavily utilised in the commutators and generators of naval vessels, with over 25,000 Metrosil units produced every week to satisfy demand. In 1940, the Journal of Scientific Instruments stated how Metrosil’s fundamental properties had been utilised in many other applications, including: providing overload protection for electrical instruments; giving increased sensitivity to voltage regulators; as a voltmeter shunt to give a contracted scale at the higher readings; producing harmonics and “peaky’’ wave forms; producing non-linear potential dividers; producing constant voltage valve circuits.

The Metropolitan-Vickers Electrical Co. Ltd., or Metrovicks as it was commonly known, always knew that the utility of Metrosil was endless. New applications would always present themselves and in 1945, the Journal of Scientific Instruments provided an update on this diverse surge diverter. Alongside an image illustrating mounting methods new applications were detailed: “The uses of these resistors, apart from surge suppression, lightning surge protection, and others mentioned in the earlier notice, include the protection of current transformers against accidental open circuit, bridge circuits balanced at one voltage only, bridge circuits giving a constant output voltage substantially independent of input voltage, the simultaneous increase of sensitivity and compensation of temperature effects in voltage sensitive relays and voltage regulators, the protection of electrolytic condensers against excess voltage, the suppression of arcing and sparking at relay contacts under certain conditions, and the operation of battery charging relays.”

Mounting Methods for Metrosil in 1945

In the 1960s, Metrosil was featured in Practical Television magazine. An article had been put together to answer questions that had been received from several readers, all of whom were interested in Metrosil’s uses and operation. The article gave three applications for Metrosil in a television set: to improve EHT regulation, to boost a first anode potential and to assist in maintaining a linear response in the cathode circuit of a video amplifier. The article concludes by confirming that these applications must not be considered exhaustive, as in the field of electronics generally, Metrosil has a host of applications. This stands true to this day – Metrosil is still discovering the next generation of applications and is still manufactured in Trafford Park, Manchester.

The Metrosil manufacturing process remains the same (well, almost the same). In the 21st century not only have Metrosil’s de-excitation products been re-designed to meet the needs of today’s power networks, but a new in-house testing capability has been introduced that allows them to be tested to 100% of their specification. Metrosil’s High Energy Test Laboratory (HETL) is rated up to 2,000 Volts, 8,000 Amps and 2 Megajoules and can run very high levels of energy through de-excitation units – levels far beyond those likely to be encountered in their operating environment. This assures generator OEMS and utilities that their assets are in safe hands. Metrosil is trusted by OEMs such as ABB, Siemens and GE.

Metrosil’s High Energy Test Laboratory

Today, not only will you find Metrosil protecting critical networks such as The London Underground, but you will find it within a range of scientific experiments covering subjects from dark matter to fusion. Over the last six years, its silicon carbide varistors have been discovered by the likes of CERN and international fusion energy companies. For CERN, it’s a case of Metrosil being rediscovered; in 1981, M&I Materials was actually supplying varistors to CERN for a pulse modulator. Fast forward to 2016 when the company decided to take part in the annual trade visit to CERN for UK companies, arranged by the Department of International Trade.

A CERN Customer Specification from 1981

The trade visits match up representatives from UK companies that would like to supply goods or services to CERN with project managers who are likely to need their products or expertise.  M&I Materials Commercial Manager, Dominic Coll, held a meeting with Magnet Designer, Glyn Kirby, and it transpired CERN was already familiar with Metrosil. 

“When Dominic mentioned the Metrosil varistor, it was a light bulb moment,” says Glyn, “and he gave me two units to play with for HiLumi.”

Glyn worked with the Metrosil technical team to tweak the design of the Metrosil varistor to meet his needs. He added: “Electrical systems normally fail when high voltage causes sparks.  It takes 20-30 milliseconds to remove the current from a superconducting magnet and that’s more than enough time to cause damage.  The varistor is designed to reduce the voltage by about 30%, while maintaining the same energy extractions as seen with high voltage designs – making the circuit much safer, robust and durable.”

Dominic commented: “M&I Materials already supplies its fire-safe MIDEL transformer fluids, Apiezon high-vacuum greases and Wolfmet tungsten heavy alloys to CERN and has done for some years – but the Metrosil reference from 1981 was in our archives.

“It stands to reason with the nature of project work and of course retiring staff on both sides, it was there waiting to be rediscovered. We’re thrilled to not only have a proven use case for Metrosil at CERN, but to support the type of ground-breaking application we’ve always been known for.”

Glyn has since fitted the varistors to the new MCBRD CCT corrector magnets that are being tested for the LHC HiLumi upgrades, the device could be easily retrofitted to other superconducting magnets to provide similar protection. Glyn has been so impressed by the device that he introduced Metrosil to colleagues at accelerator labs around the world.

In 85 years of production, Metrosil has always found its way into applications that require high energy and protection. When it was first developed, it was understood that the list of applications it could serve to protect would constantly expand – and it continues to grow to this day. With low carbon energy projects being put in place to support a green economy and combat climate change, who knows where Metrosil will find itself next?


Metropolitan-Vickers Electrical Co., Ltd. 1940 J. Sci. Instrum. 17 166
Metropolitan-Vickers Electrical Co., Ltd. 1945 J. Sci. Instrum. 22 77