The Company strives to provide a reliable and efficient electricity service. The electricity supply which is available through a standard commercial or residential outlet is subject to disturbances such as transients, electrical noise, and outages. There is very little that can be done in the design of an electric utility system to eliminate these disturbances. While older technologies were unaffected, most of the modern equipment in the home and office contains electronic circuits which are extremely sensitive to these disturbances. Customers should therefore install suitable devices to protect their equipment from possible malfunction or damage. The type of device selected will depend on the equipment that is to be protected, usage patterns, and individual needs.
» Allowable Voltage Variations
» Transient Voltages
» Common Causes of Power Problems
» Protection of Electrical Equipment
» Protection of Motorised Equipment
ALLOWABLE VOLTAGE VARIATIONS
The supply voltage provided by the Company is subject to a +/- 6% variation due to load conditions. The allowable voltage limits measured at the service entrance for different configurations supplied by the Company are shown in Table 1.
The Company should be notified in cases where the voltage measured (rms steady state) at the service entrance is outside of the values shown in Table 1.
In addition, as a general rule, the voltage drop between the customer’s load ends and any branch circuit should not exceed 5% of the supply voltage. This is part of the customer’s installation and any correction of voltage drop which may become necessary within the installation is the responsibility of the customer.
|Nominal configuration ||Minimum Voltage ||Max. Voltage || |
|115V, 1 phase, 2w, L – N ||109 ||122 || |
|115/230V, 1 phase, 3w, L - N ||109 ||122 || |
|115/230V, 1 phase, 3w, L - L ||218 ||244 || |
|115/200V, 1 phase, 3w network, L-N ||109 ||122 || |
|115/200V, 3 phase, 4w, L - N star ||109 ||122 || |
|115/200V, 3 phase, 4w, L - L star ||188 ||212 || |
|115/230V, 3 phase, 4w, open delta, L1, L2 - N ||109 ||122 || |
|L - L ||218 ||244 || |
|L3 - N ||188 ||212 || |
|230/400V, 3 phase, 4w network L-N ||218 ||244 || |
|L - L ||376 ||424 |
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Transient voltages are brief voltage fluctuations outside of the nominal supply voltage which can last up to a few milliseconds. Transient voltages, including voltage spikes or impulses, surges, swells and sags can be caused by lightning or switching loads. It has been estimated that well over 80% of all transients are generated on the customers' premises by their electrical equipment. Unlike the sustained over or under voltage condition, specialised instruments are required to capture, measure or record these transients because of their extremely short duration.
Electronic equipment is sensitive to transient voltages and should be suitably protected. The magnitude of the transient voltage and whether or not it causes damage depends on several factors. These include the amount of load switched, the point in cycle at which switching occurs, equipment design, integrity of wiring connections and grounding system, and adequacy of equipment protection from transients. The Company does not accept responsibility for damage caused by transient voltages.
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COMMON CAUSES OF POWER PROBLEMS
Any type of switching or step change in current through an impedance will produce a voltage impulse. Thus switching of the power grid, switching or cycling of building loads such as air conditioners, refrigerators, freezers, laser printers, photocopiers, elevators and arc welding equipment will give rise to transients.
A lightning strike can directly or indirectly transfer some of its energy into delicate circuitry by injecting transients via telephone, power lines, external antennas and data cables.
Poor grounding and/or bonding
When improperly grounded, the risk of malfunction or damage to electronic equipment increases due to increased noise, circulating ground currents, high neutral-ground voltage etc.
Besides being a fire hazard, faulty, loose or corroded wiring is associated with arcing, overheated terminals, damaged insulation, and transients. There is an increased risk of damage to equipment during switching due to the high resistance termination. The Company recommends that customers have a qualified electrician service their installations at least every five years.
Waveform Distortion (Harmonics)
The way in which non-linear loads draw their current can distort the steady state voltage waveform and may affect sensitive loads. Some examples of non-linear loads are switching power supplies found in most electronic equipment, arc welding equipment, 3 phase rectifiers, fluorescent ballasts, and high intensity discharge lighting. When analysed, the distorted waveform consists not only of the main 50 Hz frequency but several components at higher (harmonic) frequencies. If this is causing a problem within your facility the following should be considered: -
In a 3-phase 4 wire system which supplies mostly non-linear loads, while the even harmonics cancel each other out in the neutral, the odd harmonics are additive. Thus even when the load is fully balanced, a significant neutral current (sometimes higher than the phase current) may exist. This can cause the neutral conductor to overheat if it is not adequately sized. When supplying non-linear loads, it is recommended that the 3 phase 4 wire branch circuits should have a separate neutral for each phase conductor and feeder neutrals should be sized double that of the phase conductors.
Since it takes energy to supply current at these harmonic frequencies the available capacity of the transformer to supply the load is reduced and overheating may occur since the energy associated with these harmonic frequencies does no useful work and is dissipated in the form of heat. Thus the transformer either has to be derated or a specially designed k-rated transformer used.
Motors normally have a high starting current (as much as 3 times the running current) which can cause voltage drops and affect sensitive equipment. It is best to avoid this by placing sensitive equipment on separate circuits.
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The main purpose of grounding is safety - protection of personnel, equipment and buildings from faults. The ground is also used as a reference for logic circuits.
An effective path to ground is required to meet the following requirements:
1. To be permanent and continuous.
2. To have the capacity to conduct fault current safely.
3. To have sufficiently low impedance to limit the voltage to ground and to facilitate the operation of protective devices in the circuit.
The GEED inspects grounding as part of its facility inspection before issuing a certificate.
The Company recommends single point grounding where all equipment is referenced to the same potential (that of the earth) at the service entrance only so that any rise in ground potential appears equally on all circuits. This should be taken into consideration when installing additional ground rods for telephone, television and audio equipment. They should be tied in with the main ground electrode to avoid dangerous circulating ground currents during lightning.
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PROTECTION OF ELECTRONIC EQUIPMENT
Transient Voltage Surge Suppressor
This is the most basic protective device which is required by all electronic equipment such as the TV, VCR, stereo, microwave oven and home computer. Most suppressors contain MOV’s which clamp transient voltages to a specific value and divert the excess energy to ground. To operate efficiently a good (low resistance) ground is required. Models listed to UL’s 1449 rating at 330V in 3 modes with surge protection indicators are recommended. Avoid choosing the temporary power tap which is basically a multi-outlet extension cord.
Constant Voltage Stabilizer (Voltage Regulator)
This device regulates output voltage to near constant value for a wide input voltage swing. It is useful in environments which are subjected to frequent voltage fluctuations such as cycling motor loads, etc. Some models include filtering and surge suppressors and are known as power conditioners.
Uninterruptible Power Supply (UPS)
This device will maintain supply to the load in the event of an outage by using back up battery power. It is the appropriate choice when a power interruption can result in loss of critical data and can be used to maintain power long enough for a back-up generator to start or for systems to be shutdown in an orderly manner. Technologies may differ but they are divided into two main classes (1) - standby - where the load is fed from the mains until the power fails when the UPS is switched in and (2) on-line where the load is fed from the UPS inverter at all times and ‘sees’ the mains power only when the UPS is in maintenance mode or has a fault. Most (but not all) UPS have built in surge suppression and power conditioning to protect themselves. A UPS that does not have built in surge suppression features should be protected with a separate transient voltage surge suppressor.
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PROTECTION FOR MOTORISED EQUIPMENT
Motorised installations for refrigeration, air conditioning and other power equipment should be equipped with suitable under voltage devices to protect against sustained undervoltage or service interruption. Adequate protection should also be provided for over current. Three (3) phase motors should be equipped with suitable loss-of-phase protection devices to prevent single phase operation, improper direction of phase rotation and excessive heating due to over current. The Company will not be responsible in any way for damage to the customer’s equipment due to the failure of the customer to provide adequate protection.
Precautions to taken with the use of 60 Hz equipment
Motorised equipment designed for 60 Hz will generally run hotter, less efficiently and have a shorter lifespan when operated at 50 Hz.
Refrigerators, freezers, air conditioners and other similar equipment stamped 50/60 Hz usually are derated at the lower frequency and have two sets of voltage ratings which should be followed. A typical 50/60 Hz 14 cu. ft. refrigerator would be rated as follows in Table 2. It is recommended that a suitable step-down transformer be installed to reduce the supply voltage to that recommended by the manufacturer at 50 Hz.
|Rated frequency ||60 Hz ||50 Hz || |
|Maximum voltage ||127 V ||115 V || |
|Minimum voltage ||115 V ||105 V || |
|Rated Input ||190 V ||158 V |
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