CHRIS HALLIDAY: To protect and serve, part one
The Los Angeles Police motto is also a good one for sparkies. In the first of two articles, Chris Halliday examines the more obscure types of protection against harm from electricity.
For the electrician and electrical contractor, such protection helps to avoid personal or company fines, loss of licence or damage to individual and company reputation if something goes wrong.
Much of the protection requirements for installations, circuits and electrical equipment is detailed in the Wiring Rules, AS/NZS3000.
These rules are all about ensuring safety, having been based on experience, and are called up in legislation (the rules are law). So, in addition to a moral obligation to comply, there’s a legal obligation.
In this article we investigate the requirements and practical implications for the more obscure protection issues of undervoltage, overvoltage, impulsive transient and unbalance (voltage and current).
Power supplies in rural areas are more affected by power quality problems than those in urban areas, so there is a greater need for protection.
Rural powerlines are more exposed to lightning, storms, bushfires, trees and fauna. They are also constructed with longer spans between poles, therefore wires are more easily knocked together.
Although Australia has a 230V nominal voltage, the supply is rarely at 230V.
Undervoltage is when the supply is low for a sustained period – in power quality terms, that means longer than one minute – and delivers less than 90% of the nominal, that is, below 207V.
Undervoltage can occur for several reasons, including:
- poorly designed systems with long runs of undersized cables that might be overloaded;
- poorly set or faulty voltage regulation on the grid supply;
- burnt supply connections; or
- loss of a phase on the high-voltage grid (what could be called a ‘brownout’).
The vulnerability of appliances depends on the level of undervoltage and the type of equipment. (A power supply recently ordered for a computer monitor is rated 100-240V, so it is unlikely to be affected.)
However, motors will be badly affected by sustained undervoltage, with increased heating due to increased losses. This may result in catastrophic failure of the motor if it is unprotected by thermistors, suitably set overload protection or undervoltage/phase fail protection. The extent of the problem will depend on how mechanically loaded the motor is.
The Wiring Rules at Clause 2.8.1 require undervoltage protection where loss and subsequent restoration of voltage or drop in voltage could cause a danger to persons or property. However, protection is not required if damage to electrical equipment is considered an acceptable risk.
What is an acceptable risk and who makes this call? Often the sparkie makes the decision, but this is an issue to discuss with the client.
An acceptable risk might include situations in which the risk of fire to other parts of a building are highly unlikely, and perhaps whether the equipment is ‘throw away’ if damaged. The term ‘acceptable risk’ suggests that some form of risk assessment is to be carried out.
Failure to discuss omitting protection equipment and not conducting a risk assessment could leave the sparkie footing the bill for any damage. A court case may ensue if the owner seeks compensation for damage and lost production.
The Wiring Rules deals with overvoltage, such as that caused by lightning or switching operations.
These events are generally discussed in microsecond timeframes and not the sustained overvoltage being discussed in this section.
Sustained overvoltage occurs when the supply is high for typically longer than one minute and is more than 10% above nominal, that is, above 253V.
Overvoltage is a common problem. It will degrade most electrical equipment over time and shorten its life. The most noticeable effect is that halogen downlights blow too soon. The more insidious effects will go largely unnoticed.
The causes of overvoltage are both grid and installation based.
Grid distribution transformers are generally set to provide adequate voltage at peak loads. At light loads they may deliver too high a voltage. Also, problems can occur in the grid, such as a faulty voltage regulator or loss of a phase on the high voltage (this often causes low voltage but high voltage can occur).
Customer generation, such as by photovoltaic systems, needs a higher voltage to push excess current out into the grid. Long low-voltage runs to an installation will cause voltage rise, as the generated current must push out through the cable impedance. Effects may be localised to the installation yet they may be transferred to neighbours, depending on the size of cables and generator, and the grid layout.
The Wiring Rules are generally silent on protection for sustained overvoltage. However, there are some generic clauses:
1.6.1 requires the installation design to function properly and safely;
1.7.1 stipulates that electrical equipment be selected and installed to operate safely; and
1.6.4 deals with the utilisation voltage of the installation.
The Wiring Rules cover events caused by lightning or switching operations/
These events are labelled as overvoltage but power-quality people generally call them ‘impulsive transients’.
Lay people talk about surges and surge protection, but these terms have no real technical basis. We will use the correct terminology of impulsive transients.
Part 1 of the Wiring Rules specifies at Clause 22.214.171.124 that protection for impulsive transients is not a requirement, but measures are stated that can be used if an electrical installation is protected against ‘overvoltage’ (see Clause 2.7).
Clause 2.7 also discusses overvoltage protection measures for an insulation fault between the electrical installation and a higher-voltage circuit and also for resonant phenomena. What the … !
Well, the former requires adequate insulation screening – or segregation of circuits and transformers with adequate insulation, screening or separation of windings for transformers.
For the latter, the Rules are silent. Resonance occurs when the reactance of capacitors equals that for inductors, and transient or sustained overvoltages can occur at a particular frequency. Most things have a resonant frequency but resonance won’t be an issue unless that frequency occurs.
What does this mean for a local sparkie? Probably not much, unless you are involved in industrial plants using capacitors and/or high-voltage transformers. This would be a complete topic in itself.
Although the Rules do not require impulsive transient protection, guidance is given at Appendix F on how to select and install the right protection if required.
You would generally consider impulsive transient protection:
- if lightning is prevalent;
- in outer-suburban or rural areas, including exposed sites perhaps on a hill or in an open paddock;
- if the installation is connected via long overhead powerlines; and
- if sensitive electronic equipment is used.
Protection for voltage or current unbalance (also called imbalance) is not mentioned in the Wiring Rules, but this doesn’t mean it is unimportant.
Voltage unbalance is regarded as any differences in the three-phase voltage magnitudes and/or a shift in the phase separation of the phases from 120 degrees. It is generally expressed in percentage terms, and there are simple – and quite complex – methods for calculating it.
A voltage unbalance is likely to cause current unbalance in three-phase motors of at least six to 10 times the voltage unbalance. This is particularly so for lightly loaded motors.
Voltage unbalance is generally caused by the uneven connection of single-phase loads, either in the installation or out on the grid. Other causes include blown capacitor fuses on power factor correction equipment or photovoltaic inverters on one phase.
Voltage unbalance causes increased three-phase motor losses, reduced motor efficiency and increased motor running costs. Increased motor losses result in increased heating and loss of motor insulation life. Effective torque and speed may be reduced and motor noise increased.
If voltage unbalance is too large, then it will be important to trip a motor to protect it from the increased heating. However, many take the risk and don’t use any of the protection options.
Many motor protection relays are set to operate when the current unbalance reaches 30%. For lightly loaded motors, this may be a nuisance in some rural areas. Bypassing or cancelling this protection may be an option, but it negates the motor warranty.
Motors can be de-rated but this may be practical only if the motor is oversized. If current unbalance protection is causing nuisance trips, it may be appropriate to downsize an oversized motor and this will reduce the amount of current unbalance. Of course, the best solution is to fix the cause of the voltage unbalance.
Generally, if voltage unbalance is greater than 1%, three-phase motors will need to be de-rated. The saving grace here is that a large percentage of motors are oversized for the task and may not cause noticeable problems.
Overvoltage, undervoltage and voltage unbalance are generally managed by the same protection equipment – on a whole installation, on a single piece of equipment or at the socket outlet.
For the whole installation, a protection relay can be used with a contactor in the incoming supply cables (installed complying with Rules).
For three-phase equipment, phase reversal and protection for loss of phase may be included, and for generators there is protection for under frequency and over frequency.
Some manufacturers supply a single-pole relay that attaches to the side of their miniature circuit breakers (MCB) and operates the MCB for undervoltage conditions, so no contactor is needed.
For individual pieces of hard-wired electrical equipment, the same options can be used as for a whole installation.
For single-phase, plug-in equipment, you could use socket outlets with inbuilt undervoltage protection, but these do not generally include overvoltage protection. Years ago, powerboards were available some with undervoltage protection but they seem to have dropped out of the market.
The more expensive motors are likely to have a specialist motor protection relay that will guard against various issues including phase fail, phase reversal, undervoltage and overvoltage, and voltage and current unbalance.
If surge (I hate the term) protection is needed, then it should be installed on the incoming supply and at the sensitive electronic equipment. Surge diverters help but surge filters are much better. This is also a topic for another day.
The power quality of the incoming supply may not be perfect, so protection may be required to ensure safety and protect property.
This kind of protection may disrupt production in plants and factories, but safety should always be the overriding issue.
Problems with the incoming supply may include sustained undervoltage, overvoltage and voltage unbalance – and much faster problems such as impulsive transient events caused by lightning or load switching (what the Wiring Rules call ‘overvoltage’).
The Rules are silent on protection options for sustained overvoltage and voltage unbalance, other than with generic clauses about the design and safety of installations. Undervoltage protection, according to the Rules, is a risk management decision.
It will pay to discuss protection options with clients, or problems could and you may be held accountable.