FYI Inrush Current

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  1. There has been much discussion in the last few months regarding high inrush currents being associated with high frequency electronic ballasts. Following is a technical overview of the subject.

    What is it?
    Devices with solid state power supplies, such as computers, copiers, and electronic ballasts, as well as many magnetic devices such as motors, drives, and core & coil ballasts, have an input current during initial start-up that can be several times greater than their operating or steady-state current. This current during start-up is generally referred to as Inrush Current. For High Frequency Electronic Ballasts, this current during start-up typically lasts for much less than 1/2 of a 60 Hz cycle (< 8 msec).
    What are the effects?
    High current conditions can affect electrical system components. The main area of concern is the tripping of circuit breakers and fuses. If the circuit breaker or fuse is not designed to handle the amount of inrush current that is present, the device could trip upon energizing the circuit or during circuit operation.

    It has been suggested that during turn-on, momentary contact bouncing in the switch or relay may cause the contacts to become pitted due to arcing between the contacts points. This can be present in all systems, and is not a direct result of inrush current. However, the higher the overall system current, the faster contact deterioration may occur.

    Since inrush current is only present during initial system energization, it is not a factor during system turn-off.
    What amount is present in your lighting system?
    Inrush current is present in both magnetic and electronic ballasts. The amount of ballast inrush current varies across manufacturers, ballast types, and ballast brands. In addition, the inrush current of a complete lighting circuit is affected by the total source impedance of the entire distribution system. A system with a low impedance can deliver a greater amount of inrush current to the ballast(s) than a circuit with a high impedance.

    The system impedance is determined by several electrical distribution system variables. These variables include the impedance of the main transformer; the distance of the lighting circuit to the main transformer; the type and size of wire between the branch circuit and transformer; the wire size and wire type of the lighting branch circuit, and the length of the wires in the lighting branch circuit. These variables determine the maximum amount of current that can be delivered to the ballast(s) at the moment of turn-on.

    Electronic ballasts are generally characterized into two groups, those with an active front-end, and those with a passive front-end. The term front-end refers to the power input section of the ballast.

    Generally, electronic ballasts with active front ends have Total Harmonic Distortion below 10%, and passive front ends typically have THD below 20%. However, due to multiple circuit designs, and the continuous design changes that are evolving, this may not always be the case.

    The active ballasts typically have low impedance during start-up, due to the need to charge the system circuitry. Many passive ballasts typically have an inductive choke on the front-end, which has a higher impedance, resulting in a lower inrush current. Active electronic ballasts can have inrush currents as high as 100 times or more its operating current, with a duration of up to .8 milliseconds. Passive electronic ballasts with an input choke can have an inrush current of up to 30 times operating current, with a duration up to 5 milliseconds. This is compared to magnetic ballasts that have an inrush current of up to 10 times the operating current, with a duration of under 10 milliseconds.

    Based on the assumption of inrush current being 100 times the magnitude of the steady state current, a 20 amp circuit loaded to 16 amps could have an inrush current as high as 1600 amps (16 x 100). However, due to system impedances the total system inrush will probably not reach the theoretical maximum calculated.

    Previous laboratory testing of sample ballasts has shown an inrush current of 75 amps on a 120V, 20 amp system loaded to 16 amps with two-lamp active electronic ballasts. Total duration was approx. 5 milliseconds. These lab results show a significantly lower circuit inrush current, with a greater duration, than the theoretical calculations for a single ballast predict. These results are only valid for one type of ballast in a controlled environment, and extrapolation to any other location is not possible without first investigating all system parameters. All things considered, the likelihood of achieving the maximum inrush is probably very slight.
    What can you do?
    When planning a new or retrofit ballast installation, take the following steps in order to reduce potential inrush current problems.

    First, determine the inrush current drawn by the ballast you have selected. A range may be supplied by the manufacturer due to the various system impedances encountered in laboratory testing.

    Second, calculate the theoretical maximum inrush current for the circuit based on the circuit values for your specific installation (wire size, length, etc.).

    Third, select a fuse or circuit breaker that is capable of handling the inrush current for the duration of the inrush. Typically, fuses and breakers are rated to handle inrush currents that are several multiples of their steady-state ratings.

    Fourth, select switches and contacts that are able to withstand the inrush current. All switches and contacts in the circuit, both locally and at the electrical or lighting panel, should be properly sized. Contact your switch manufacturer to determine the proper unit. If occupancy sensors or other control devices are to be used on the circuit, contact the sensor manufacturer to ensure compatibility with the type and number of ballasts being controlled.


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