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While three-phase networks are the norm almost everywhere in Europe, a wide variety of different network configurations and voltages are used in North America. Both the North American mains voltages and network configuration affect the selection and use of switchgear and protection equipment.
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While the term operating voltage is used as an umbrella term in IEC standards (comparable to nominal system voltage or rated voltage in North America), a distinction is made in North America between different types of voltage.
The service voltage is the voltage in the supply network. Observing the tolerances of the service voltage is particularly essential at the transfer point between the lines used by the electric utility companies and the lines used by the end user, i.e. at the point of connection (point of common coupling). The actual service voltage is usually between 95% and 105% of the nominal voltage.
The utilization voltage is the voltage in the consumer network: In particular, it refers to the effective voltage at the point of connection for the apparatus. In the worst case scenario, the utilization voltage may range between 87% and 106% of the nominal voltage. The difference between the minimum service voltage and the minimum utilization voltage is the permissible voltage drop within the consumer installation.
In terms of tolerances, a distinction is made between an ideal level (range A) and a tolerable level (range B) for both voltages. For more detailed definitions, see ANSI C84.1, the national standard for utility voltage tolerances in North America. This standard defines the nominal voltages and operating tolerances for 60-Hz electrical systems between 100 V and 1200 kV. The American National Standards Institute—ANSI—facilitates and coordinates the US voluntary standards and conformity assessment system. The Canadian equivalent of this US standard is CSA CAN3-C235-83.
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As is the case in Europe, in North America, the rated voltage at which the apparatus was designed to operate is often specified on the rating plate for motors, motor switchgear and motor-protective equipment (voltages for electric-motor apparatus are defined in the NEMA standard MG 1 and in the NEC).
The rated voltage for motors is also referred to as the nameplate voltage. However, this voltage is not the same as the nominal mains voltage. For example, a motor wound for 460 V can be connected to a 480-V network. The nameplate voltage roughly corresponds to the minimum service voltage and takes into account that the motor apparatus is almost never operated with the nominal voltage of the network. This allows the operating currents for adjusting the protection equipment to be determined more precisely and the permissible voltage tolerances of the motors to be better utilized. In catalogs from switchgear manufacturers and on rating plates for switchgear and protection equipment, either the nameplate voltage (simpler for the user) or the nominal system voltage (mains voltage) are specified.
The North American networks are usually softer than European networks, as the transformers often have a higher short-circuit voltage of up to 7%. The deviating short-circuit voltage should be taken into account when calculating the short-circuit current for larger consumer installations. When the power transformer has a higher short-circuit voltage, the maximum short-circuit current it provides is lower than if it had a lower short-circuit voltage. In IEC standards, tables containing short-circuit currents for transformers usually only list the short-circuit currents provided by the transformers on the secondary side. Higher currents are sometimes specified in US tables, as additional components of motor recovery currents are taken into account, which are also supplied by the short-circuit network.
North American network configurations and their frequency may also differ significantly compared to those in Europe in some cases. Network configurations in the US can rule out the option of using various protective measures against electric shock; they also determine whether a neutral conductor is present in the network, and whether it is grounded or ungrounded.
The TN-S network described in IEC standards is very similar to the "three-phase wye" star-point earthed network. In this network, voltages of 277/480 V are used in the US, while voltages of 347/600 V are used in Canada.
Phase-grounded delta networks are also in use in North America—this type of network configuration is practically unknown in European distribution networks. This network does not have a neutral conductor and the mains voltages are 480 V in the US and 600 V in Canada. There are two types of this network in use: In an "ungrounded delta" network, the transformer is not grounded on the secondary side. However, in a "corner-grounded delta," network, one phase conductor is directly grounded.
The "three-phase wye" and "delta" network configurations are particularly relevant for industrial machinery applications.
In North America, "split-phase" networks are almost always found within households and small office buildings. The name "split phase" results from the fact that the center tapping of a transformer winding is grounded—meaning the voltage between the center tapping and either phase is exactly half of the voltage between the two phases. In this network configuration, the voltage is 120 V or 240 V.
The "high-leg delta" network configuration is a mixture of "delta" and "split-phase." This means that this network can provide three voltages: 120 V, 208 V and 240 V. "High-leg delta" networks are mainly used in large office buildings and small industrial buildings.
In the US, mains voltages of up to 480 V 60 Hz are most common; in Canada, mains voltages of up to 600 V 60 Hz are used. For machine exporters, determining the network configuration at the operating location often presents a major problem.
When exporting electrical systems and equipment, it can therefore be useful to make sure the system is able to operate using the local network, irrespective of whether there is a neutral conductor; this can be achieved by installing an input transformer in the switchgear system. This approach enables single-phase apparatus to be connected to its own single-phase network using a neutral conductor. When selecting switchgear and protection equipment for three-phase apparatus, it should be noted that switchgear and protective devices designed in accordance with IEC or EN standards are—due to their clearance and creepage distances—only partially approved for use in solidly grounded electrical systems with or without a neutral conductor.
If there are any doubts about the network configuration, alternatives must be used that can also be operated in delta networks at full voltage. If the limitation only affects a few devices, you can, for example:
Anyone who wants to export devices, machines or systems to the US or Canada must deal with the network configurations and mains voltages that are used there, because they can be considerably different from European networks. These aspects must be considered to ensure reliable operation and to meet the standards applicable in North America. If the network configuration cannot be clearly identified, it is recommended that it is clearly indicated that the machine or system is supplied for a solidly grounded star network (e.g. 480 Y/277 V) in the quotation.
Since not all switchgear and protection equipment can be used with the high voltage of 600 V, it is also often recommended to use input transformers. The Eaton white paper "Voltage specifications and network configurations relevant for export in North America" provides detailed information on this topic.
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