Fuses are typically installed in safety switches. Separately mounted fused safety switches are typically categorized as general-duty and heavy-duty types.
The general-duty type safety switch is rated at 240 V maximum and is typically used in residential and light commercial and industrial applications. The heavy-duty type safety switch is rated at 600 V maximum and is typically used in commercial and industrial applications.
Safety switches can typically be ordered with neutral assemblies and equipment grounding assemblies. There is currently no listing for safety switches that are to be used specifically with nonlinear loads.
It is recommended that the manufacturer be contacted to determine if oversized neutral assemblies can be installed in safety switches serving nonlinear electronic load equipment without voiding any listing requirements. In addition, the manufacturer should be contacted to determine if an isolated equipment grounding bus can be installed in the safety switch enclosure for those applications that require this grounding configuration.
Whenever fuses are utilized, there is a risk of a single-phasing condition if one fuse on a three phase system blows. Safety switches are generally not stored energy devices, and may not contain auxiliary functions such as undervoltage release or shunt trip attachments that help protect against a single-phasing condition.
This is an important consideration because some three phase electronic load equipment may be susceptible to damage if a single-phase condition persists. Other devices may need to be installed to provide proper single-phasing protection.
Blown fuse indicators
Recommended practice is to use blown fuse indicators for the quick and safe determination of the source of power outage affecting downstream electronic load equipment. Some safety switches and fused circuit breakers contain indicating devices located on the front enclosure that indicate a blown fuse condition. Some fuses contain an indicator light, providing visual indication that a fuse is blown.
Interrupting ratings
Interrupting ratings of new fuses or existing fuses should be evaluated to determine if proper interrupting ratings are applied. Interrupting ratings need to be reevaluated if there are any changes to the power system, such as installing K-factor transformers.
These transformers are typically specified or manufactured with a low impedance (%Z) resulting in a higher available short-circuit current on the secondary. This condition can be a problem especially where low interrupting capacity fuses, such as Class H fuses, are installed (Class H fuses have an interrupting rating of only 10 000 A).
WHY WORK PERMITS? - PRINCIPLE BEHIND WORK PERMITS BASIC INFORMATION
Safe systems of work are crucial in work such as the maintenance of chemical plant where the potential risks are high and the careful coordination of activities and precautions is essential to safe working. In this situation and others of similar risk potential, the safe system of work is likely to take the form of a permit to work procedure.
The permit to work procedure is a specialized type of safe system of work for ensuring that potentially very
dangerous work (e.g. entry into process plant and other confined spaces) is done safely. Although it has been developed and refined by he chemical industry, the principles of permit to work procedures are equally applicable to the management of complex risks in other industries.
Its fundamental principle is that certain defined operations are prohibited without the specific permission
of a responsible manager, this permission being only granted once stringent checks have been made to ensure that all necessary precautions have been taken and that it is safe for work to go ahead.
The people doing the work take on responsibility for following and maintaining the safeguards set out in the
permit, which will defi ne the work to be done (no other work being permitted) and the timescale in which it must be carried out.
To be effective, the permit system requires the training needs of those involved to be identified and met, and monitoring procedures to ensure that the system is operating as intended.
Permit systems must adhere to the following eight principles:
1. wherever possible, and especially with routine jobs, hazards should be eliminated so that the work can
be done safely without requiring a permit to work
2. although the Site Manager may delegate the responsibility for the operation of the permit system, the overall responsibility for ensuring safe operation rests with him/her
3. the permit must be recognized as the master instruction which, until it is cancelled, overrides all
other instructions
4. the permit applies to everyone on site, including contractors
5. information given in a permit must be detailed and accurate. It must state:
(a) which plant/equipment has been made safe and the steps by which this has been achieved
(b) what work may be done
(c) the time at which the permit comes into effect
6. the permit remains in force until the work has been completed and the permit is cancelled by the person
who issued it, or by the person nominated by management to take over the responsibility (e.g. at
the end of a shift or during absence)
7. no work other than that specifi ed is authorized. If it is found that the planned work has to be changed,
the existing permit should be cancelled and a new one issued
8. responsibility for the plant must be clearly defined at all stages.
The permit to work procedure is a specialized type of safe system of work for ensuring that potentially very
dangerous work (e.g. entry into process plant and other confined spaces) is done safely. Although it has been developed and refined by he chemical industry, the principles of permit to work procedures are equally applicable to the management of complex risks in other industries.
Its fundamental principle is that certain defined operations are prohibited without the specific permission
of a responsible manager, this permission being only granted once stringent checks have been made to ensure that all necessary precautions have been taken and that it is safe for work to go ahead.
The people doing the work take on responsibility for following and maintaining the safeguards set out in the
permit, which will defi ne the work to be done (no other work being permitted) and the timescale in which it must be carried out.
To be effective, the permit system requires the training needs of those involved to be identified and met, and monitoring procedures to ensure that the system is operating as intended.
Permit systems must adhere to the following eight principles:
1. wherever possible, and especially with routine jobs, hazards should be eliminated so that the work can
be done safely without requiring a permit to work
2. although the Site Manager may delegate the responsibility for the operation of the permit system, the overall responsibility for ensuring safe operation rests with him/her
3. the permit must be recognized as the master instruction which, until it is cancelled, overrides all
other instructions
4. the permit applies to everyone on site, including contractors
5. information given in a permit must be detailed and accurate. It must state:
(a) which plant/equipment has been made safe and the steps by which this has been achieved
(b) what work may be done
(c) the time at which the permit comes into effect
6. the permit remains in force until the work has been completed and the permit is cancelled by the person
who issued it, or by the person nominated by management to take over the responsibility (e.g. at
the end of a shift or during absence)
7. no work other than that specifi ed is authorized. If it is found that the planned work has to be changed,
the existing permit should be cancelled and a new one issued
8. responsibility for the plant must be clearly defined at all stages.
BEST LOCATIONS FOR PANEL BOARDS AND SWITCH BOARDS OF ELECTRONIC EQUIPMENT BASIC TUTORIALS
Where should panel boards be located?
Switchboards and panelboards that support electronic load equipment and related loads should be properly designed and installed. Recommended practice is to use panelboards specifically listed for nonlinear loads if they serve electronic load equipment.
As a minimum, panelboards should be rated for power or lighting applications, and should not be a lighterduty type. Special attention should be given to the location and installation methods used when installing panelboards.
In addition, protective devices shall adequately protect system components, neutral buses should be sized to accommodate increased neutral currents due to harmonic currents from nonlinear electronic load equipment, and equipment ground buses should be sized to accommodate increased numbers of equipment grounding conductors due to the recommended practices of using insulated equipment grounding conductors and dedicated circuits for electronic load equipment.
Surge protective devices may also be installed external to, or internal to, the switchboards or panelboards.
Location
Panelboards that serve electronic load equipment should be placed as near to the electronic load equipment as practicable, and should be bonded to the same ground reference as the electronic load equipment.
Other panelboards located in the same area as the electronic load equipment that serve other loads such as lighting, heating, ventilation, air conditioning, and process cooling equipment should also be bonded to the same ground reference as the electronic load equipment.
Panelboards should be directly mounted to any building steel member in the immediate area of the installation. Isolation of a panelboard from the metallic building structure by an electrically insulating material, as an attempt to prevent flow of high frequency current through the panelboard, is not recommended practice.
The panelboard and metallic building structure, separated by a dielectric material, become capacitively coupled. The capacitive coupling presents a low impedance at high frequency defeating the original purpose.
NFPA 780-1997 requires effective grounding and bonding between objects such as structural building steel and a panelboard located within side-flash distance (approximately 1.8 m (6 ft), horizontally) of each other. Insulation materials, commonly used in an attempt to separate a panelboard from building steel, are rarely capable of withstanding lightningi nduced arcing conditions.
Switchboards and panelboards that support electronic load equipment and related loads should be properly designed and installed. Recommended practice is to use panelboards specifically listed for nonlinear loads if they serve electronic load equipment.
As a minimum, panelboards should be rated for power or lighting applications, and should not be a lighterduty type. Special attention should be given to the location and installation methods used when installing panelboards.
In addition, protective devices shall adequately protect system components, neutral buses should be sized to accommodate increased neutral currents due to harmonic currents from nonlinear electronic load equipment, and equipment ground buses should be sized to accommodate increased numbers of equipment grounding conductors due to the recommended practices of using insulated equipment grounding conductors and dedicated circuits for electronic load equipment.
Surge protective devices may also be installed external to, or internal to, the switchboards or panelboards.
Location
Panelboards that serve electronic load equipment should be placed as near to the electronic load equipment as practicable, and should be bonded to the same ground reference as the electronic load equipment.
Other panelboards located in the same area as the electronic load equipment that serve other loads such as lighting, heating, ventilation, air conditioning, and process cooling equipment should also be bonded to the same ground reference as the electronic load equipment.
Panelboards should be directly mounted to any building steel member in the immediate area of the installation. Isolation of a panelboard from the metallic building structure by an electrically insulating material, as an attempt to prevent flow of high frequency current through the panelboard, is not recommended practice.
The panelboard and metallic building structure, separated by a dielectric material, become capacitively coupled. The capacitive coupling presents a low impedance at high frequency defeating the original purpose.
NFPA 780-1997 requires effective grounding and bonding between objects such as structural building steel and a panelboard located within side-flash distance (approximately 1.8 m (6 ft), horizontally) of each other. Insulation materials, commonly used in an attempt to separate a panelboard from building steel, are rarely capable of withstanding lightningi nduced arcing conditions.
K - RATED TRANSFORMERS BASIC INFORMATION AND TUTORIALS
What are K-Rated Transformers?
UL and transformer manufacturers have established a K-factor rating for dry-type power transformers to indicate their suitability for supplying nonsinusoidal load currents. The K-factor relates a transformer’s capability to serve varying degrees of nonlinear load without exceeding the rated temperature-rise limits.
The K-factor is the ratio of stray losses in the transformer winding for a given nonsinusoidal load current to the stray losses in the transformer winding produced by a sinusoidal load current of the same magnitude. These transformers are typically specially designed to handle the increased heating effects and neutral currents produced by nonlinear electronic load equipment. The following are some of the design features:
a) The neutral bus is rated at 200% of the secondary full load ampere rating to accommodate the large neutral currents that principally result from triplen harmonics and phase imbalance. The transformer neutral bus rated at 200% is capable of accommodating oversized or multiple neutral conductors.
b) The winding conductors are specially configured and sized to minimize heating due to harmonic load currents. Special configurations and sizing such as multiple, parallel conductors can reduce the skin effect of the higher frequency harmonics and accommodate the balanced triplen harmonics that circulate in the transformer primary (delta) windings.
c) Cores are specially designed to maintain flux core density below saturation due to distorted voltage waveforms or high line voltage. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. The K-factor for a linear load is 1.
For any given nonlinear load, if the harmonic current components are known, the K-factor can be calculated and compared to the transformer’s nameplate K-factor (refer to
As long as the load K-factor is equal to or less than the rated K-factor of the transformer, the transformer is suitably rated and is considered safe to operate at rated load without overheating. Typical load K-factors for facilities containing large numbers of computers appear to range between 4 and 13.
Measured K-factor on the secondary of step-down transformers that serve almost exclusively nonlinear loads, such as personal computers, have been observed to range as high as 20, but this is extremely rare. In most cases, a transformer with a K-factor rating of 13 can be sufficient to handle typical nonlinear electronic load equipment.
UL and transformer manufacturers have established a K-factor rating for dry-type power transformers to indicate their suitability for supplying nonsinusoidal load currents. The K-factor relates a transformer’s capability to serve varying degrees of nonlinear load without exceeding the rated temperature-rise limits.
The K-factor is the ratio of stray losses in the transformer winding for a given nonsinusoidal load current to the stray losses in the transformer winding produced by a sinusoidal load current of the same magnitude. These transformers are typically specially designed to handle the increased heating effects and neutral currents produced by nonlinear electronic load equipment. The following are some of the design features:
a) The neutral bus is rated at 200% of the secondary full load ampere rating to accommodate the large neutral currents that principally result from triplen harmonics and phase imbalance. The transformer neutral bus rated at 200% is capable of accommodating oversized or multiple neutral conductors.
b) The winding conductors are specially configured and sized to minimize heating due to harmonic load currents. Special configurations and sizing such as multiple, parallel conductors can reduce the skin effect of the higher frequency harmonics and accommodate the balanced triplen harmonics that circulate in the transformer primary (delta) windings.
c) Cores are specially designed to maintain flux core density below saturation due to distorted voltage waveforms or high line voltage. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. The K-factor for a linear load is 1.
For any given nonlinear load, if the harmonic current components are known, the K-factor can be calculated and compared to the transformer’s nameplate K-factor (refer to
As long as the load K-factor is equal to or less than the rated K-factor of the transformer, the transformer is suitably rated and is considered safe to operate at rated load without overheating. Typical load K-factors for facilities containing large numbers of computers appear to range between 4 and 13.
Measured K-factor on the secondary of step-down transformers that serve almost exclusively nonlinear loads, such as personal computers, have been observed to range as high as 20, but this is extremely rare. In most cases, a transformer with a K-factor rating of 13 can be sufficient to handle typical nonlinear electronic load equipment.
STANDBY GENERATOR SYSTEM FOR ELECTRONIC EQUIPMENT BASIC INFORMATION AND TUTORIALS
Incompatibility issues regarding emergency standby generator systems and downstream electronic load equipment are gaining more recognition in modern power systems. UPS systems and electronic load equipment can be very susceptible to voltage waveform distortion and frequency variations.
The distortion of the voltage waveform is primarily a function of the magnitude and harmonic content of the load current and the impedance of the upstream electrical distribution system. Standby generator systems generally have a much higher impedance than the utility system.
Therefore, the voltage waveform distortion typically increases when loads are fed by standby generator power. One of the most common incompatibility situation is with generator systems and downstream UPS systems.
These situations can range from problems with the UPS inverter trying to synchronize to the static bypass circuit to the UPS input failing to accept the input voltage and thus causing the UPS system to go to battery power.
In this latter condition, the voltage distortion typically improves when the load is fed from battery power (the load is now on battery and not acting on the impedance of the generator system) and the UPS input accepts the line voltage. Once again, voltage distortion can increase when the loads are powered via the UPS system, and the UPS
a) Isochronous electronic governor to regulate frequency. These governors typically maintain frequency regulation within 0.25% of the setting, as opposed to approximately 3% for mechanical governors.
b) Permanent magnet excitation system or filtering means to isolate the voltage regulator
power circuit from the distorted waveform.
c) Generators with a 2/3 pitch stator winding design to minimize third harmonic waveform distortion.
d) Low subtransient reactance to minimize voltage waveform distortion. cycles back and forth on battery power. Recommended practice is to provide the standby generator manufacturer with information on the type, rating, and characteristics of the electronic load equipment.
Many generator manufacturers and UPS manufacturers have guidelines for sizing emergency generators when supplying UPS systems. This rating will typically depend on the type and size of the UPS system.
In general, the standby generators should have the following characteristics to minimize adverse interactions when supplying nonlinear loads.
The distortion of the voltage waveform is primarily a function of the magnitude and harmonic content of the load current and the impedance of the upstream electrical distribution system. Standby generator systems generally have a much higher impedance than the utility system.
Therefore, the voltage waveform distortion typically increases when loads are fed by standby generator power. One of the most common incompatibility situation is with generator systems and downstream UPS systems.
These situations can range from problems with the UPS inverter trying to synchronize to the static bypass circuit to the UPS input failing to accept the input voltage and thus causing the UPS system to go to battery power.
In this latter condition, the voltage distortion typically improves when the load is fed from battery power (the load is now on battery and not acting on the impedance of the generator system) and the UPS input accepts the line voltage. Once again, voltage distortion can increase when the loads are powered via the UPS system, and the UPS
a) Isochronous electronic governor to regulate frequency. These governors typically maintain frequency regulation within 0.25% of the setting, as opposed to approximately 3% for mechanical governors.
b) Permanent magnet excitation system or filtering means to isolate the voltage regulator
power circuit from the distorted waveform.
c) Generators with a 2/3 pitch stator winding design to minimize third harmonic waveform distortion.
d) Low subtransient reactance to minimize voltage waveform distortion. cycles back and forth on battery power. Recommended practice is to provide the standby generator manufacturer with information on the type, rating, and characteristics of the electronic load equipment.
Many generator manufacturers and UPS manufacturers have guidelines for sizing emergency generators when supplying UPS systems. This rating will typically depend on the type and size of the UPS system.
In general, the standby generators should have the following characteristics to minimize adverse interactions when supplying nonlinear loads.
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