Man-made disasters and resilient home power systems

September 11th, 2015, Published in Articles: EE Publishers, Articles: Vector


Mike Rycroft

Mike Rycroft

Resilient power systems are usually taken to mean grid power systems that can survive natural disasters and provide power to critical sites under extreme conditions. This requirement emerged in the USA after Hurricane Katrina and is now being legislated into many states. In South Africa, we are not subject to such devastating natural disasters as tropical storms, hurricanes, earthquakes and tsunamis, but we have to face regular and personally devastating man-made disasters known as load shedding.

The first response to this disaster, back in 2008 – 2009, was the rush to install small diesel or petrol generators in the range of anything between 5 and 20 kVA, on residential installations. A standby generator is an essential item for a business and, if you can afford to have it installed and maintained properly, can be a life saver.

The same cannot be said for generators installed at residential premises. Besides the fact that they are noisy and need constant attention to fuel and batteries, they were often installed by the homeowner in a manner which can only be regarded as downright unsafe. The main problem is that the standby generator is connected directly into the fixed wiring installation, a situation which produces a number of problems, mainly related to protection and earthing.

The second stage involved a domestic UPS-type arrangement, but did not find much acceptance due to the fact that a huge bank of lead-acid batteries was used, and it also used the fixed wiring. These units further required the wiring to be split into essential and non-essential loads, which caused problems of its own.

Load shedding has not decreased over the years and we are told that it is likely to continue for several more years. Industry, however, has come up with an elegant solution to the problem. Standard UPS units used in the IT industry are designed for short back-up times, have appropriately small batteries, and are unsuitable for long-term backup use.

A new family of end-user UPS-type unit called “inverters” for some reason, has lead to a new generation home standby power units in the 1 – 2 kW range. These are designed for long-duration operation by the inclusion of high charge rate, intelligent battery chargers, and allow incorporation of long-duration batteries which can be recharged rapidly.

The adoption of these units has been assisted by the development of LED lighting, which now comes in a variety of shapes and sizes, with standard fittings. Although still fairly expensive, they offer the homeowner the ability to light up all the important areas of the home without using much power, and can be connected to an emergency power unit via socket outlets without entering the permanent wiring. LED units in the 2 – 4 W range can provide adequate light for interiors and even exteriors. If you consider that ten such units will consume 20 – 40 W, the lighting load for a house, added to the load of the TV, internet and computers – all plug-in units – is probably less than 1 kW.

The ingenious techno-boff now merely needs to think like an interior decorator and place standard lamps and other free-standing fittings around the house strategically and string extension cords to TV and ITC equipment. The inverter is always in the loop and the “emergency” circuits become part of the standard wiring of the house.

The first unit of this type which I came across consisted of a 2 kW Mecer inverter fitted with a settable 10 or
20 A charger, plus batteries, mounted in a wooden cabinet which was not out of place inside the house ( see Fig.1).

Fig 1: Home power backup system providing plug- in solution to load shedding.

Fig 1: Home power backup system providing plug- in solution to load shedding (Credit: Chris Rycroft)

The usefulness of this solution has not been lost on entrepreneurs, and I have since seen some real disasters offered on the market which I would not even consider for my workshop. There are also units appearing in custom-made “designer” cabinets, and the industry is getting wise to the fact that a unit of this type must look good to be acceptable. DIY kits comprising the inverter and batteries are now sold at hardware outlets. The big advantage of these systems is that everything connects via plugs and sockets, and a single domestic socket can handle the whole system.

Running flexible cables all around a modern house is probably not an acceptable practice and more elegant solutions are required so that the final product may resemble permanent wiring with flexible cable disappearing into roof spaces etc.

The question is, how long before the SABS gets wind of this and claims that such installations constitute fixed wiring and must comply with the standard and have a CoC? I served on that body for several years when I was still gainfully employed and can remember the attention focused on what was called “pre-wired plug-in installations” such as mobile offices. The ruling at that stage was that the pre-wired installation would either have to comply with the wiring code or with an appropriate appliance safety standard and could not just be plugged in.

Industry should nevertheless be congratulated for coming up with such an elegant and workable solution to an abominable problem, and it is hoped that development of such units continues.

Perhaps we could even hope that, as load shedding is really a form of non-voluntary demand-side management, the purchase of these systems could be subsidised.

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