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The UL 508A and NFPA 79 standards, which are requirements for the approval of devices and systems in North America, both repeatedly refer to "feeder circuits" and "branch circuits."
These terms describe the two types of main circuits, which are rather unfamiliar in the world of the IEC.
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The National Electrical Code (NEC) defines "feeder circuits" as all wires and devices in electrical circuits between the various types of energy supply and the feed side of branch circuit overcurrent protective devices (BCOPDs). The system is considered using the load in the system in the direction of the voltage source. The feeder circuit therefore includes all devices positioned after the load side right up to the final branch circuit protective device (BCPD). "Feeder circuits" may also be described as power distribution circuits.
According to the NEC, branch circuits are all wires and components positioned after the final overcurrent protective device. The branch circuit therefore runs from the load side through to the first branch circuit protective device (BCPD). Branch circuits can be main circuits with a variety of loads, including motor loads and heating or lighting loads. Additional protective devices can be installed between the BCPD and the load. For example, the BCPD can be the protective device for a group of manual motor controllers/manual motor protectors (motor-protective circuit breakers).
The motor-protective circuit breakers (in this case without any additional terminals) are then positioned between the load (motor) and the BCPD and will serve as additional overcurrent protective devices.
Having knowledge of the dividing line between feeder circuits and branch circuits is vital for European machine builders to be able to select the correct components for their equipment. Our video gives you a short introduction.
Only devices with large clearance and creepage distances to the main current connections may be used in feeder circuits.Large clearance and creepage distances must also be present between the main current terminals on one side, the auxiliary current or control current terminals on the other side and for earthed parts.
As a general rule, industrial control devices in accordance with UL 508/CSA C22.2 No. 14-05 (small clearance and creepage distances), such as contactors, are not permitted to be used in feeder circuits. For example, this also applies for supplementary protectors in accordance with UL 1077/CSA C22.2 No. 235-04 (R2009).
But now—driven by the semi-conductor industry—contactors are being accepted as the main protectors for safety switches (functional safety) in feeder circuits, provided that the required large clearance and creepage distances are maintained between the main circuits, the mounting plate and adjacent components. Devices with "normal" clearance and creepage distances—similar to those outlined by the IEC—may be used in branch circuits. Of course, devices with large clearance and creepage distances may also be used in branch circuits.
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In principle, the following can be used as branch circuit protective devices: listed fuses in accordance with UL 248, circuit breakers in accordance with UL 489 and manual and combination motor controllers (type E/F) in accordance with UL 508.
However, a differentiation must be made betweenBCPDs for universal use and those for special use (e.g. those exclusively for motors). It is also important to note that BCPDs must be tested and approved for use with certain operating equipment, meaning there is a limited selection for equipment such as frequency converters.
When selecting and using UL-listed fuses, it is necessary to consider whether components have been tested and approved by the manufacturer, and if so, with which types of fuse. With standard testing conditions, only the breaking capacity of the fuse is specified. But there are also testing conditions with fuse types which must be specified as a special solution, with the fuse type and amperage displayed on the device. When using UL 508 type E motor starters as branch circuit protective devices, there are various limitations to consider: Common errors include using type E motor starters for transformer protection or for protecting heating systems.
For example, both types of equipment may only be protected by circuit breakers in accordance with UL 489 or by fuses in accordance with UL 248. The switching capacity of the BCPDs or the SCCR of UL 508 type E or type F motor starters must always be high enough for the respective application. Under certain circumstances, the switching capacity must be as high as the switching capacity of an upstream current-limiting protective device, because the let-through current is not considered for UL 508A switchgear systems between protective devices connected in series.
To ensure the electrical equipment of a machine is fit for export to North America, it is necessary to understand how main circuits are broken down into feeder and branch circuits. Otherwise, the machine may not be approved in North America due to insufficient clearance and creepage distances in the feeder circuit or the use of unsuitable BCPDs, for example using miniature circuit breakers in accordance with UL 1077 or motor-protective circuit breakers in accordance with UL 508 instead of branch circuit protective devices. Tips on how to avoid typical errors in the electrical equipment of machines for export to North America can be found in Eaton's whitepaper of the same name.
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