Technical Bulletin

Technical Bulletin
September 1, 2005
Zero Crossing/Inrush & Voltage Transient
The Importance of Zero Crossing
Electronic ballasts are a common energy saving component of modern lighting systems. As the use of
these ballasts becomes more widespread, lighting control technologies must keep pace.
It is evident that the failures of some lighting control units, such as occupancy sensors, have been
related to the inrush current of certain types of electronic ballasts. While inrush current is not the sole
contributor to these failures, it is a principal cause. To reduce high inrush current levels, zero crossing
circuitry was developed to provide a solution. Watt Stopper/Legrand’s zero crossing circuitry provides
zero voltage switching, a switching technology that reduces potentially harmful inrush current levels.
Inrush current
Inrush current occurs when the electronic ballast is first switched on. Almost all electronic ballasts utilize large capacitors and inductors to store electric energy. This energy is used to provide consistent,
high frequency output power to the lamp. Most types of electronic ballasts utilize a capacitor that must
be initially charged when the switch is first turned on. This initial charging process is what creates the
inrush current. Often, uncontrolled inrush can be 50-100 or more times the ballast’s normal operating
current. High inrush current typically exceeds the current limits of the relay, which often is designed to
handle only up to 10 times the normal operating current.
Inrush current and the lighting system
The high current peak of inrush current can have adverse effects on the lighting system. It can affect
the relay contacts, the circuit breaker, and other related components. Repeated exposure to the stress
of inrush current can shorten the operating life span of these elements (e.g. fusing of relay contacts
due to excessive heating from inrush and subsequent switch failure). The amount of damage done to
the controlling switch from inrush is proportional to the square of the magnitude of the inrush current.
Factors determining inrush
The magnitude of inrush current is determined by three important factors. The impedance of the building lighting system itself, the type and number of electronic ballasts used, and the design and technology of the lighting control switch.
The AC power system of a building is usually fixed, as it is designed to have low resistance to prevent
voltage drop and wasted energy. The building power system only limits extremely high levels of inrush
current and cannot be relied upon to limit normal inrush from electronic ballasts.
The type of electronic ballast also plays a key role in determining inrush level. High Power Factor (HPF)
Watt Stopper/Legrand • 2800 De La Cruz Blvd • Santa Clara CA 95050 • 800.879.8585 •
Watt Stopper/Legrand Technical Bulletin
Issue: #TB105.1 • Date: September 1, 2005
ballasts are the most common electronic ballasts used. High power factor electronic ballasts consist of
active and passive power factor correction models. Both use electrolytic capacitors that generate inrush
current. Low power factor (LPF) ballasts, used with compact fluorescent lamps (CFLs), have a similar
capacitor and also experience high levels of inrush current.
Lighting control technology and design also affects the inrush level in a lighting system and can be practical for preventing potential inrush problems. The components of the switch, including the relay contact
design and contact closure time are important elements of determining inrush capability. Zero crossing
circuitry represents a means for reducing the negative effects of inrush levels through technology at the
control level.
Theoretical background & ballast technology
Before describing the effects of zero crossing circuitry on inrush, it is important to clarify certain fundamental concepts. Zero crossing circuitry, which provides zero voltage switching, works because of the condition of AC power. AC power is transient and not sinusoidal steady state during the initial switching. This
assumption sets up the condition for zero voltage switching to reduce inrush current, as demonstrated
with proven empirical and computer model data.
Active power correction
In active power factor correction ballasts, the capacitor is in a series circuit that consists of a fast rectifier,
a “boost” inductor, a full-wave rectifier, and the two-winding EMI filter inductor. This can be expressed as a
three component series circuit. The components of the
circuit are a single inductor (L) equal to the sum of the
boost inductor and the leakage inductance of the EMI
inductor, a resistor (R) equal to the sum of the inductors’ winding resistances, and the electrolytic capacitor (C). The series L-R-C circuit of this type will have a
basic half-period (T) equal to π times the square root
of the product of L and C. In addition, the “characteristic impedance” (Z) of the L and C is equal to square
root of L over C.
277V Active Power Factor ballast model
Basic Half-period: T = π (LC)1/2
Characteristic Impedance: Z = (L/C)1/2
If T is small compared to the half time period of 60Hz (8.33mS), and the AC voltage is relatively stable, then
a good approximation of the peak current to charge the capacitor is denoted by the voltage (V) at the
instant of turn-on divided by (Z + R). The peak current is inrush current.
Peak Current (Inrush Current): Ip =
Theory then shows that inrush current when the AC voltage line is near its peak will be many times higher
than the inrush near zero voltage. Any inrush that still exists near zero voltage is caused by the fact that
AC voltage does not remain at zero, and, as the voltage rises, the capacitor starts charging.
Passive power factor correction
The passive power factor corrected ballasts possess a similar L-R-C circuit as the active ballasts, yet the
Watt Stopper/Legrand • 2800 De La Cruz Blvd • Santa Clara CA 95050 • 800.879.8585 •
Watt Stopper/Legrand Technical Bulletin
Issue: #TB105.1 • Date: September 1, 2005
values of the components are different. With passive
power factor, the value for L (inductor and leakage
impedance) is hundreds of times larger than the active
power factor ballast’s L. A larger value for L means a
longer half-period time T and a lower peak current.
The much larger L means a much higher Z, and thus a
many times lower inrush current. Even at the phase
angle for the worst case of inrush, the amount of inrush
generated by the passive power factor ballast is minimal.
277V Passive Power Factor ballast model
Low power factor
The most common use for low power factor ballasts
(less than 30 watts) has been for compact fluorescent
lamps. In low power factor ballasts, the capacitor is
charged from a full-wave rectifier directly from the AC
line. A small EMI series inductor is used, with the L in
the L-R-C series much smaller than active power factor ballasts. The C is also smaller, due to the lower
wattage. As with the active ballasts, the maximum
inrush occurs near the peak of the AC line.
277V Low Power Factor ballast model
Based on the component values of the L-R-C series
circuit in both the active and low power ballasts and basic electronics theory, the maximum inrush can be
expected when the AC voltage line is near its peak, and that inrush will be substantially less for zero voltage switching.
Zero crossing circuitry and inrush
Zero crossing circuitry provides zero-voltage switching capability. Normal switching occurs at random
points along the AC voltage line. Statistically, a significant number of inrush current peaks occur at or near
worst-case. Zero voltage switching is synchronized to the AC voltage, so that switching occurs at or near
zero volts. Switching at or near zero volts will ensure minimized inrush current and increased operational
life of the lighting control components. With zero voltage switching, each switch event will have the same
low inrush current.
Inrush Current Levels
Inrush Current (Amps)
75 Degrees
90 Degrees
Active PFC
Passive PFC
Phase Angle
Watt Stopper/Legrand • 2800 De La Cruz Blvd • Santa Clara CA 95050 • 800.879.8585 •
Watt Stopper/Legrand Technical Bulletin
Issue: #TB105.1 • Date: September 1, 2005
Computer model simulations, as well as actual ballasts measurements have demonstrated that zero voltage switching could reduce inrush generated by multiple ballasts by a factor of four or five. Zero voltage
switching substantially reduced inrush in all active power factor ballasts tested from the 75 to 90 degree
phase angle cases. Passive power factor ballasts, which typically have low inrush levels, keep inrush current low regardless of phase angle.
Zero crossing and voltage transients
Another potential hazard to the lighting system are the voltage transients. The voltage transient is caused
by the interruption of the current flowing in the electronic ballast’s inductor at switch-off. The transient
energy can potentially be over 1000 volts. The actual amount of voltage depends, like inrush, on a number
of system factors, but is proportional to the amount of inductance and the current in the inductor at switch
off. Voltage transients can arc through wire insulation, to other wires or even the ballast or ballast inductor. Arcing such as this will eventually destroy part of the lighting system.
Electronic ballasts with passive power factor designs, by virtue of their large inductors, have the highest
potential risk for switch-off voltage transients. Because the wave form is mostly sinusoidal and high power
factor, the AC current is for the most part in phase with the AC voltage. If a zero voltage switch is
employed, the voltage will be switched off at or near zero and the AC current will thus be at or near zero.
Zero voltage switching thus allows only very low transient voltages to occur at switch off.
The use of certain types of electronic ballasts exposes the lighting system to the effects of inrush current
and voltage transients. These two phenomena, both products of the design and function of the electronic
ballast, have the potential to do damage to the components of the lighting control as well as the ballast
and other parts of the lighting system. Zero voltage switching, as evident from testing and computer modeling, minimizes the effects of inrush current and transient arcing throughout the lighting system.
• Active power correction and low power factor ballasts both exhibit high levels of inrush current. Zero crossing circuitry reduces the effects of inrush by switching when AC voltage is at
or near zero, preventing damage from the AC current that can occur during the inrush current
• Passive power correction ballasts, while not generating a damaging amount of inrush current,
possess a greater risk for voltage transient arcing during switch off. This arcing can damage
not only elements of the relay but can also harm the ballast as well. By switching off at or
near zero voltage, the risk of voltage transients is greatly reduced.
Watt Stopper’s zero crossing circuitry, through zero voltage switching, provides longer operational product
life and improved reliability for lighting control devices, as well as protection for the rest of the lighting
system from the stress of repeated exposure to inrush current and voltage transients.
Refer to Technical Bulletins TB 106.1 and 107.1 for further discussion of inrush and zero crossing.
Watt Stopper/Legrand • 2800 De La Cruz Blvd • Santa Clara CA 95050 • 800.879.8585 •
Watt Stopper/Legrand Technical Bulletin
Issue: #TB105.1 • Date: September 1, 2005
Actual Ballast Measurement - 2 Lamp, 277V
Inrush Current at 90˚
Phase Angle
35.4A at approx. 400V
Inrush Current at 0˚
Phase Angle
3.48A at approx. zero V
Watt Stopper/Legrand • 2800 De La Cruz Blvd • Santa Clara CA 95050 • 800.879.8585 •