Surges or overvoltage can wreak havoc on industrial electrical process equipment and, as a rule, the more sensitive the equipment the more likely it is to be damaged. But damage to equipment and the resulting cost of repairs might not be the biggest cost. Production downtime can cost a company thousands of dollars a minute.
Preventing such losses is all about understanding the cause of many of these failures and when it comes to overvoltage the effects might not be obvious unless the surge is a big one. Often a surge (or even several) does not always damage a piece of equipment. However, long term exposure to overvoltage eventually causes component failures. In such cases the failures tend to go unexplained or are attributed to ‘fair wear and tear’ because there seems to be no other reason. It may also be too difficult or time-consuming to find the cause of the fault to explain it.
However, help is now at hand. Damage can be prevented by fitting the correct type of anti-surge equipment. Knowing the basics of this technology and how to implement it can lead to another revenue stream for switchboard builders as well as contractors in the industrial sector and can save installation owners a lot of grief and money.
Surge protection devices (SPDs) comprise a range of single and three-phase devices designed to limit the impact of various severities of electrical surges likely to arise in various parts of an installation. SPDs are not like filters, which only eliminate particular frequencies or ‘noise’.
SPDs are likened to seat-belts in cars, which do not come into play until you are involved in a crash. Unprotected electrical systems are also an accident waiting to happen but, better than seat-belts, buckling up with SPDs will actually prevent accidents from occurring.
Dealing with the likelihood of surge damage comes down to a simple risk assessment. By evaluating the likely frequency, severity and consequence of overvoltage incidents it will soon become clear whether the cost of either an SPD retrofit or the inclusion in a new project or switchboard design will outweigh the ongoing cost of replacing damaged equipment and losing valuable production time.
Australian executive of German SPD maker Dehn + Söhne, Hans Slagter, says the main causes of overvoltage or electrical spikes are not lightning strikes but less dramatic surges generated within the boundaries of industrial electrical systems and by electricity supply companies switching loads or experiencing faults.
“These can be protected against by utilising the appropriate SPDs within the plant’s electrical and control systems.
“Spikes and transient voltages can also be due to direct or remote lightning strikes but the likelihood of a direct lightning strike (cloud-to-ground) is not that common as New Zealand mostly has a very low lightning flash density.
“The second type, a remote or distant lightning strike, is a little more frequent. This type strikes electricity networks’ overhead lines or pylons elsewhere or can be a relatively nearby cloud-to-cloud strike that emits a lightning electromagnetic pulse (LEMP). The resulting LEMP generated by a remote lightning strike can still induce high transient voltages into unprotected systems a fair distance away.”
Lightning is the most destructive form of naturally occurring high voltage (HV) transients and a direct lightning strike can cause millions of volts to be generated at currents up to 200 kiloamps (kA) and can induce transient voltages of several thousand volts in circuits within two kilometres of the strike itself. Even if grounded quickly it can still cause damage or kill livestock standing near earth electrodes due to step-voltage electrocution.
The MetService say around 45,000 lightning strikes a year terminate to ground in New Zealand with an average peak current of 26,000 amps. Fortunately, lightning strikes only exist for a few milliseconds (ms) duration and high capacity SPDs have been designed to withstand these high currents and voltages and divert surges to earth.
More common causes
Electrical network-borne transient voltages arise through the normal day-to-day operation of the electricity distribution network such as when substations are switched into the system to boost supply capability or occur when parts of the network are switched back on after maintenance or repairs. They are also caused by accidents such as vehicles hitting power poles and overhead lines banging together in the wind. These incoming mains-borne disturbances can also be negated by SPDs at the incoming supply point.
Slagter says New Zealand and Australia have a non-mandatory lightning protection standard for designing lightning protection systems. AS/NZS 1768 Lightning Protection gives guidelines for protection of persons and property from lightning hazards. It covers conventional lightning protection systems with lightning conductors, down-conductors, earthing systems and SPDs. The standard also provides a risk assessment process for determining what lightning protection level must be applied if any.
The electrical wiring standard AS/NZS 3000 also provides some informative, non-mandatory details about SPDs in appendix F. Amendment 2 (December 2012) of the standard is now more up to date as previously a 32 amp fuse was recommended incorrectly to provide overcurrent back-up protection for a 40 kA SPD. This has now been amended to a value recommended by the supplier, because a 32A fuse can ‘blow’ if a surge pulse of ≥ 7kA occurs and would disconnect the SPD from the system.
Most electricians come across SPDs only when they are fitting a new commercial or industrial switchboard with over a couple of hundred amps capacity. More and more these SPDs are specified to comply with international standards from the IEC 61643 suite such as part 11, surge protective devices connected to low-voltage power systems – requirements and test methods. Many specifiers and board builders consider the local standard out-dated, less than rigorous and not in keeping with mainstream practice and international trends.
Slagter says the risk assessment of lightning damage in AS/NZS 1768 is a simplified version of the IEC standard.
“Our local standard references the IEC 62305 standards suite, protection against lightning, around seventeen times. However, the local standards development committee seems reluctant to adopt the IEC lightning and surge protection standards, although most of our trading partners have done,” he says.
“AS/NZS 1768 is due for revision and New Zealand seems to favour adopting the IEC standard, which is the trend with new standards. But Australian local standards development committee members favour American UL standards despite the US moving towards IEC standards so they can export their products internationally. What we need is a product-based standard, like IEC 61643-11 which specifies tests procedures and parameters for various SPD so compliant products can all compete on a level playing field.”
Danger within
The most frequent surges occur within industrial electrical installations as a consequence of work being carried out. Problematic industrial transients such as power factor capacitor banks being switched in and out of circuit, starting up motors or transformers and operating sporadic high current-use devices like welders as well as the operation of fuses or other circuit protection, all add to the risks of overvoltage damage.
Simon Johnston of Rockwell Automation says it is not only mainstream electrical systems that can be damaged by surges.
“Of equal importance in industrial applications are communications and data systems. Just because these operate at lower voltage than mains-powered equipment, it does not mean they are immune from surges – the lower the voltage, the lower the surge immunity.
“While power supplies for data and telecommunications have their own low-voltage convertors, these can be compromised by mains-borne surges that ‘get past’ or wreck power supplies and damage the semiconductors inside. Fortunately, there are also SPDs specifically for low voltage and communications equipment. While most electrical SPDs are parallel-wired, data and communications units are typically directional series-wired devices.”
He says if the failed equipment or component is critical to the process it may require a shutdown to repair or replace it. However, the damage may occur again if the correct SPD is not selected and installed to protect it.
Switchboard designers offer SPDs and power quality metering options but older boards can be retrofitted with metering that logs times and duration of surges. This enables process control staff to assess if surges have caused failures.
SPDs should ideally be installed during the original installation as they only form a small part of the cost of a new installation. However, more electrical staff and managers of manufacturing operations are now undertaking risk assessments and looking at retrofitting SPDs because of persistent and costly failures of process control equipment.
Geoff Thomson the New Zealand manager of German surge equipment supplier Weidmuller says industrial electricians are increasingly aware that surges can kill PLCs and drives and stop processes.
“The issue is not so much the cost of fitting SPDs but the cost of downtime to the client. If a surge in a food process takes out a single PLC it could mean having to dump tens of thousand dollars’ worth of ruined product. Electricians aware of the consequences of surges and other power quality issues can now do a lot to protect industrial processes and keep production running,” he says.
In Whitianga, electrical contractor Chris Brown kept having to replace a particular variable speed drive that was burning out repeatedly at the local mussel processing plant. He contacted Thomson and between them identified surge problems were the cause and sorted them out with SPDs.
Automated processes are most at risk from surges and Thomson says the increasingly complex nature of process control means more and more sensitive equipment such as sensors, actuators, low voltage power supplies, PLCs and the like are exposed to overvoltage damage.
“But SPDs are available for protecting individual pieces of equipment and many of these are din-rail mounting for ease of fitting.
“However, lower capacity downstream SPDs will fail to function properly if there are no higher capacity upstream devices that limit surges to levels the downstream ones can handle. This includes surge protected multi-boxes and plug adaptors which cannot handle more than a modest surge,” he says.
Cascaded protection
An overview of surge protection can be compared to cabling. Just as cables entering an installation are largest, the ones to the distribution boards are smaller and the cables to final circuits are the smallest, so it is with SPDs – the highest capacity is at the mains and the lowest on final circuits. This cascaded protection stops the heaviest surges at the entry point while the next SPDs ground any residual overvoltage.
To obtain the maximum protection for an industrial plant the surge protection system needs to cover all levels of the electrical installation including sensitive circuits and any other external electrical services entering the plant. Surge protection should also be considered for air-conditioning as without cooling, servers may have to be shut down and, because condenser units are often on the roof they can be exposed to lightning.
While most electrical SPDs are parallel or shunt connected, control, instrumentation and data/communications units are typically directional series-wired devices.
Thomson says most SPDs utilise metal oxide varistors (MOVs) to divert surges down to earth.
“These variable resistors, as their name implies, have internal resistance which is dependent on voltage. They have a high resistance at low voltages which changes to a low resistance at higher voltages which then diverts surges to earth in a matter of nanoseconds.
“MOVs are fast-responding and bi-direction and are suited to mid to high-range current discharging capacity up to 200 kA. However, MOVs age due to damage by repeated surges and are also not suited to protecting data networks as their high capacitance attenuates higher frequencies above 100 kilohertz (kHz). Most SPDs are fitted with visual indicators to show if they are working or have failed and some are fitted with relays to allow remote monitoring. Others have removable surge modules as SPDs are inherently sacrificial and may need occasional replacing.
“Some of the higher capacity SPDs designed to arrest lightning use, spark gaps rather than MOVs to ground surges. However, 95 percent of surges are not lightning related and only peak at a few kA,” says Thomson.
Hans Slagter says SPDs need to be installed upstream of any residual current device (RCD) and be protected by high rupturing capacity (HRC) fuses or circuit breakers in accordance with the manufacturers recommendations so they are automatically disconnected if they fail.
“SPD back-up protection circuit breakers or fuses should be chosen to match or exceed the prospective short-circuit fault current at the point where they are installed. The fuse size for SPDs should be verified with the supplier as too small a rating may cause nuisance tripping.
“There are three classes or types of SPDs in the European and IEC standard. The highest capacity is type one (lightning current arrester) which is for protecting the point of entry or main switchboard when the building has an external LPS. This is because higher currents, in the order of 200 kA, may have to be grounded in a lightning strike (so a good earthing system is paramount).
“Type two (overvoltage arresters) is for protecting distribution board circuits or control cabinets. Type three (overvoltage arrester) is for direct load protection and final subcircuits or terminal equipment. Type two SPDs can also be used in main switchboards where there is no external lightning protection system on the building as they have sufficient capacity to protect against indirect lightning strikes and induced surges,” says Slagter.
“The various SPD classes also have different surge definitions, Class I impulse current (Iimp) is defined by three parameters, a peak current value (Ipeak), a charge (Q) and a specific energy (W/R). These are tested according to the sequence of the operating duty test which is used for the classification of a class I SPD. These parameters (peak value, rate of current rise, load, charge, specific energy) can best be simulated by means of a 10/350 µs impulse current wave form. Typical values are 25 kA.
“Class II nominal discharge (In) is the crest value of the current through the SPD having a current waveshape of 8/20µs. This is used for the classification of the SPD for class II test and also for preconditioning SPDs for class I and II tests. Typical values are 20 kA (8/20µs)
“Class II maximum discharge current (Imax) is the crest value of a current through the SPD having an 8/20µs waveshape and magnitude according to the manufacturers specification. Imax is equal to or greater than In. Typical values are 40 kA (8/20µs).
“The difference between In and Imax is that In is a measure of the work rate, or performance of the SPD, whereas Imax gives an indication of the SPDs longevity.
Slagter says the joint Australia New Zealand standard does not use the three IEC classes of protection but refers to maximum current ratings (Imax) for five different categories of SPD. “Category A SPDs are for final sub-circuits and are rated at 3 – 10 kA. Category B SPDs are for major sub-mains, short final sub-circuits and load centres and are rated at 10 – 40 kA. Category C1 units are for low risk service entry points and rated at 40 kA. Category C2 SPDs are for service entry points fed by long overhead lines or in a large industrial or commercial premise and are rated at 40 – 100 kA. The most demanding category C3 is for service entry points for buildings in high lightning areas or ones fitted with a LPS and are rated at 100 kA.
“Most of the world including Europe has a four-part lightning protection standard, IEC (or EN) 62305 Protection Against Lightning. Part 1 covers general principles, part 2 is on risk management, part 3 is about physical damage to structures and life hazard while part 4 covers electrical and electronic systems within structures,” he says.
By comparison
Comparing SPDs from various suppliers can be challenging where different ratings are supplied on data sheets. Despite the complexity of comparing different SPDs a key parameter is the ‘clamping voltage’ which is the voltage the device lets pass into an installation. The closer the clamping voltage to the system voltage, the better it is.
There is a strong case for using higher rated SPDs than those recommended though. A higher capacity unit will handle more surges with less on-going degradation and will probably last a lot longer and give you more piece of mind.