November/December 2020

41 www.drivesncontrols.com November/December 2020 CONTROLGEAR AND ENCLOSURES n Hot disconnects: how to stop sparks flying Hot disconnects are performed so frequently in factories that we don’t give a second thought to the technologies involved. Nick Lewis, product manager at the cable and connector specialist PEI-Genesis, looks at what is involved in hot disconnecting and the factors you need to consider when choosing such connectors. I magine if you had to go to the circuit- breaker box and turn off your entire home’s electricity supply every time you needed to plug or unplug something. Now take that inconvenience and scale it up a thousand times. Imagine a large factory that may have to disconnect equipment thousands of times a day, possibly in explosive or hazardous atmospheres, and the problem becomes clear. Some of the principles involved in hot disconnecting can be seen in the design of the UK’s humble 13A plug. Some people say that the UK has the best plugs in the world, with a combination of features including built- in fuses, ground terminals and yank protection, which are found almost nowhere else in the world. The case is put in a YouTube video ( https://www.youtube.com/watch?v=UEfP1 OKKz_Q ) that has been viewed more than five million times. One particularly innovative part of the design of the 13A plug is that the earth terminal is 5mm longer than the live and neutral terminals. The ground makes contact first, providing an escape route for any excess charge before the live and neutral contacts mate a moment later. The practical upshot of this is that British plugs are far less likely to arc when being connected or disconnected. Connecting and disconnecting powered connections like this is known as hot disconnecting and, while avoiding arcing is beneficial for safety in the home, in industry the risks are far higher. The first question is: why hot-disconnect at all? If equipment is powered down before disconnecting, all of the associated problems can be avoided. For small industrial applications this is acceptable, but in factories this approach becomes impractical. Powering down equipment can lead to major losses in production and can be an extremely time-consuming and labour- intensive process to plan and implement. Many industries work this way, and the only real consequence is a reduction in the connector’s cycle lifetimes as the arcs slam repeatedly into the contacts. Arcing can lower cycle ratings by more than 500 times, so it’s certainly a factor to bear in mind, but the increased cost of replacing connectors 500 times more frequently are massively outweighed by the increased production generated by keeping equipment powered up. Another important factor is that industrial voltages are often many times higher than the 230V of domestic circuits. The possibility of arcing is a direct function of the voltage and the distance between the contacts, so a larger voltage results in more powerful and longer-lasting arcs. Because the contacts start off touching, some arcing is unavoidable at the point that they un-mate. This is limited in connectors which include a longer grounding terminal, such as the British domestic plug, but arcing will always occur to some degree. However, there are some industrial situations where even minor arcing poses dangers – most notably in explosive environments. The threat of catastrophic explosive consequences for arcing in these environments means that Atex-certified hot- disconnect connectors require an additional design consideration. Some connectors, such as the Falcon 25 range made by Trolex, use a two-stage connecting process. First the enclosure mates, and then the live contacts. This creates an explosion- containing capsule, so that if there is an explosive atmosphere present to be ignited by the arc, any resulting explosion is contained safely within the capsule and doesn’t have the opportunity to spread. Hot disconnecting equipment is something that almost every industry does, but how many plant workers think about the connectors when they are performing this action? A hot disconnect in your home probably feels natural and doesn’t have any negative consequences if you are using a safe 13A plug. But, in a factory, it will reduce connector lifespan. However, sacrificing connectors for productivity is a sensible trade-off, just one you should understand before you make it. n Most factories performmany hot disconnects daily without giving a second thought to the connector technologies involved. Performing such actions safely in hazardous environments requires well-designed connectors (right).