Unplanned power interruptions are estimated to cost large organisations over £8000/hour. Smaller companies have been known to go bankrupt, so maintaining business continuity is critical. James Hunt looks at some of the main power continuity and protection issues in terms of surge protection and UPS.

How long is it since you last experienced a mains power problem? Most of us can barely remember the last time, but over the last five years, there have been around 96 million unplanned power interruptions in England and Wales alone. It only takes a mains failure lasting as a little as a millisecond to cause serious consequences, yet in data centres, 99.999% uptime is now the target. Expensive and potentially dangerous outages can occur in industry, airports, computer centres and hospitals, so it is sensible to counter power brownouts, blackouts and other damaging problems when they occur.

The first part of this article is about protecting electrical and electronic equipment from potentially damaging power surges using surge arrestor devices. The second part concerns Uninterruptible Power Supplies (UPS), which protect against losing mains power.

Surge protection

Voltage spikes or surges are fast, short duration electrical transients (overvoltages) in an electrical circuit. They are typically caused by lightning strikes, tripped circuit breakers, short circuits, lighting switching, AC motor operation, power and transitions in other equipment on the same power line, power utility malfunctions, storm-damaged powerlines and 100kHz – 1MHz electromagnetic pulses.

The result can be cumulative equipment damage, failure, system resets, downtime and data losses. It can even result, especially from lightning strike, in catastrophic equipment failure, as well as longer-term business disruption, expensive equipment repair and replacement costs.

Surge prevention

Surge arrestors can prevent such problems by regulating the voltage supplied to electric devices and/or systems by shorting voltages to earth above a safe threshold, or by blocking very fast – a transient voltage surge suppressor protects against spikes that occur in less than 0.000000001s. Surge protection devices are known collectively as SPDs.

Parameters affecting SPD design and selection include the let-through voltage; a lower value provides greater protection but with a shorter surge arrestor life. Also important is the energy that can be absorbed without failure and response times, as there is always a slight delay – the longer the response time, the longer the connected equipment is exposed to the surge. Luckily, since voltage spikes and surges also take time to evolve, SPDs usually suppress the most damaging part in time.

Lightning strike can cause very large overvoltages and peak currents (up to 400,000A). Some SPDs designed to protect three-phase systems against lightning strike are said to be able to pass currents of 320,000A/phase (1,200,000A total). They should be energy co-ordinated so that lightning voltages are reduced to below the immunity of devices to be protected.

Surge arrestor types:

Metal oxide varistor (MOV) – This limits voltages to about three to four times the normal circuit voltage by diverting the surge current elsewhere. However, MOVs can be degraded when subjected to several large transients – or a greater number of smaller transients. Connection in parallel (in matched sets) increases current capability and life expectancy.

Transient suppression diode – This limits voltage spikes to less than twice the normal operating voltage. It has a relatively low current capability – as long as current variations stay within range, life expectancy is very long, but if ratings are exceeded, failure may occur. Applications are with circuits exposed to smaller current spikes or more frequent spikes.

Gas discharge tube (GDT) – This conducts more current for its size than others, but life is finite and only a few large transients (or a greater number of smaller transients) are accepted. The trigger time is longer, so a higher voltage spike exists before significant current is conducted; extra protection may be needed. Applications are in telecommunications equipment and HF lines.

Selenium voltage suppressor – This type is similar to a MOV but is less effective with let-through voltage, yet life is longer. The type can dissipate power continuously and retains its voltage let-through characteristics throughout the surge event. Applications are mainly in high-energy DC circuits.

Quarter-wave coaxial surge arrestor – A tuned quarter-wavelength short-circuit stub allows a bandwidth of frequencies (narrow or wideband) to be passed, while presenting a short to any other signals, especially towards DC. The type provides possibly the most reliable protection for RF signals above 400MHz, so is suitable for RF signal transmission route applications.

Combined lightning current / surge arrestor – This provides energy co-ordination between the various surge arrestor devices and the devices to be protected.

2) UPS

Lighting, telephones, mobiles, laptops, chargers, PCs, tablets, peripherals and servers all depend absolutely upon a reliable electricity supply, as do industrial processes. The answer is to use a UPS – a mature technology that is still being improved to cope with ever more difficult power demands. UPS avoid un-planned power outages, save valuable data and provide time (autonomy) to allow safe shutdown of affected equipment.

UPS represent big and growing business because of the increasing reliance on ever greater numbers of digital devices, as well as the demand for cleaner power that they require, and from the changing nature of mains power supplies themselves. For example, as older coal-fired power stations are phased-out, and as more renewable sources are installed, grids will change in ways that are not fully understood – but there will almost certainly be shortfalls. Coming smart grids will also have an effect.

UPS technologies

UPS choice is largely determined by capacity, redundancy, scalability and total cost of ownership. It is important to seek early professional advice so that the right solution is chosen. There are several types of UPS:

Offline – The simplest and least expensive static (purely electronic) UPS, this type provides limited surge and spike protection. It is the most energy efficient type because charger and inverter do not operate at all times, but an offline system can fail when most needed.

Online (double conversion) – This more expensive static type (typically 10kVA or above) is suitable for operation with a longer-term backup system (eg: a genset). The type is more reliable because the inverter runs continuously, and there is greater protection against surges, spikes, sags, brownouts, electrical noise and harmonics. However, it is relatively inefficient, especially at part load – though economy modes improve efficiency.

Line-interactive – This static type (also 10kVA or above) has better voltage stabilisation, power conditioning, energy efficiency, lower running costs and longer life than the offline version. However, less protection is provided against frequency changes and harmonics.

Rotary – This (non-static) type’s spinning flywheel provides energy storage for short-term ride-through if mains power is lost. Protection is also provided against spikes and sags. However, the flywheel typically provides only 10 – 20s of protection before it slows, so often, backup power is provided only while a diesel back-up genset starts and stabilises power. Sine wave quality, harmonics, fault clearing and high inrush load capability are all good. The type is relatively expensive and is usually used for critical applications above 400kVA.

Diesel – This comprises diesel engine, generator, kinetic energy storage module and coupling choke to provide power conditioning and ride through energy for the UPS. The diesel engine provides longer-term backup. Availability and reliability are very high, as is stand-by operational efficiency.

Emergency gensets – These provide extended run-time after the UPS has done its job of protection, but before mains power is restored. Not UPS in themselves, they are important in the overall picture.

Specification

In specifying a UPS, key factors include the business type, existing protection, equipment importance, load type, ambient conditions and future requirements. Static UPS applications are broad, while those for rotary UPS are more limited. Static UPS are typically used in homes, offices, large public buildings, and small to medium sized data centres. Rotaries are typically better suited for applications where there are many short power inrushes (eg: motor switching). Rotary UPS may also provide greater network security, especially for very large multiple megawatt data centers, and tend to be better for centralised architectures. Static UPS are better for distributed solutions.

In conclusion, changing power supplies and demands, the increasing dependence on electricity, the changing nature of digital communications, plus climate change mean that the market for UPS is certain to grow, so there will be significant opportunities for switched on electrical contractors.