John Casey 24/02/2021



OIL INSULATED RMUs: Oil RMUs have been manufactured by switchgear companies and used by customers reliably for a good 60 years or more. The expected life when sold was 25 years. There is a switchgear company still producing fused tee-off RMUs today and some customers are still buying them. Oil RMUs needed to be maintainable and required new contacts etc. after a specified number of fault makes and load breaks. In the event of a tee-off fault, the MV fuses also needed to be changed.

Today, companies are available (eg. Long Controls Ltd.) to maintain oil switchgear and after service and refurbishment the RMU is deemed to have a further 20 years or more service life, still requiring new contacts etc. as a brand new unit would require. During the refurbishment process it is sometimes possible to enhance the fault ratings of some Oil RMUs from 13.1kA to 20kA by using improved contact kits that have been proven at a short circuit testing station. Oil RMUs are still providing good service in the network, after 50+ years and maintenance. This is in line with the ‘circular economy’ – maintain, repair, reuse DON’T scrap, dispose of and waste resources.

SF6 INSULATED RMUs: SF6 insulated RMUs seriously arrived in the market around the late 1980s. Some of their advantages were, low maintenance, internal arc relief, circuit breaker protected tee-off (instead of MV fuses). As these units were ‘low maintenance’ they had enhanced electrical endurance. (These enhanced electrical performances are now standard in IEC and ENA). There was no need to be able to service the internals of these units and, in fact, it is advantageous to not have to venture into the gas enclosure as electrical arcing in SF6 produces bi-products which can only be handled by qualified personnel with special equipment. For this reason, the SF6 RMUs became sealed for life and, unlike the oil RMUs, contacts could not be changed. Their electrical life cannot be extended. Some are welded tanks that cannot be accessed, at all, in any event.

Some SF6 units also have a special filling procedure, in that the almost perfect vacuum that is required in the gas enclosure before filling with SF6 cannot be supported by the tank alone. During manufacture, the gas enclosure has to go into another chamber to lower the external differential pressure on the tank from atmospheric (1 bar) so that the tank internal pressure is always equal to or a higher pressure than its surrounding environment. This was also a cost-effective filling methodology as leak detection could be incorporated in the filling process equipment. It was also cheaper to design a tank that was only ever subject to a positive internal pres-sure. These RMUs were ‘sealed for life’ and the age of the disposable RMU had arrived, as in other industries and other consumer products. Their lives were initially expected to be 25 years or when the extended electrical endurance levels were reached. More recently 40 years of life has been requested. The units should then be disposed of safely by qualified companies.


Future RMUs will not contain SF6 for various well known reasons. Their expected life will be 40 years. It is possible that some may have alternative gases in them. However, some are likely to be ( as some are today) DRY AIR / VACUUM INTERRUPTION. With the dry air / VI solution, there is a massive opportunity to embrace the ‘Circular Economy’ ideals as other industries are. This would allow future maintenance / repair of the units as with the oil units. This is not suggesting we should go back to more maintenance than on SF6 units but just to have the opportunity to NOT scrap a unit when it has reached its designated endurance limits, but to service / refurbish it as with oil RMU s in the past. A consideration is that a sealed for life RMU consists of 6 switches / CBs, if any one of those items reaches its endurance limits the whole unit is disposed of even if the other 5 have not even been used at all. A maintainable RMU would be able to have that worn out element replaced and the RMU could continue in service. This would be in line with ‘Circular Economy’ ideals.

CONCLUSION: To allow maintenance of an RMU may result in a slightly higher purchase price, but, as proved with oil RMUs, may result in a lower lifetime cost. Now is the opportunity to influence the next generation of RMU design as the units that will be sold in 2023 are being designed now. I would welcome and be grateful for any customers opinions on this subject – FUTURE RMUS: MAINTAINABLE OR SEALED FOR LIFE?




Fourrmula Limited are pleased to advise that John Casey has been appointed the Chairman of the Beama Principal Products Technical Committee. John has been a member of the committee since 1996 when he was appointed Chief Engineer at Long & Crawford. He has also been a Beama member on behalf of Lucy Electric, P&B Switchgear Ltd., Fundamentals Ltd., and presently Long Controls Ltd. John also remains a member of the Beama Principal Products Management Committee and the Beama Networks Senior Strategy Group representing Long Controls Ltd. 28/05/2020