Electrical characteristics of lamps

Incandescent lamps with direct power supply

Due to the very high temperature of the filament during operation (up to 2,500 °C), its resistance varies greatly depending on whether the lamp is on or off. As the cold resistance is low, a current peak occurs on ignition that can reach 10 to 15 times the nominal current for a few milliseconds or even several milliseconds.
This constraint affects both ordinary lamps and halogen lamps: it imposes a reduction in the maximum number of lamps that can be powered by devices such as remote-control switches, modular contactors and relays for busbar trunking.

Extra Low Voltage (ELV) halogen lamps

• Some low-power halogen lamps are supplied with ELV 12 or 24 V, via a transformer or an electronic converter. With a transformer, the magnetization phenomenon combines with the filament resistance variation phenomenon at switch-on. The inrush current can reach 50 to 75 times the nominal current for a few milliseconds. The use of dimmer switches placed upstream significantly reduces this constraint.
• Electronic converters, with the same power rating, are more expensive than solutions with a transformer. This commercial handicap is compensated by a greater ease of installation since their low heat dissipation means they can be fixed on a flammable support. Moreover, they usually have built-in thermal protection.

New ELV halogen lamps are now available with a transformer integrated in their base. They can be supplied directly from the LV line supply and can replace normal lamps without any special adaptation.

Dimming for incandescent lamps

This can be obtained by varying the voltage applied to the lampere
This voltage variation is usually performed by a device such as a Triac dimmer switch, by varying its firing angle in the line voltage period. The wave form of the voltage applied to the lamp is illustrated in Figure N38a. This technique known as “cut-on control” is suitable for supplying power to resistive or inductive circuits. Another technique suitable for supplying power to capacitive circuits has been developed with MOS or IGBT electronic components. This techniques varies the voltage by blocking the current before the end of the half-period (see Fig. N38b) and is known as “cut-off control”.
Switching on the lamp gradually can also reduce, or even eliminate, the current peak on ignition.
As the lamp current is distorted by the electronic switching, harmonic currents are produced. The 3rd harmonic order is predominant, and the percentage of 3rd harmonic current related to the maximum fundamental current (at maximum power) is represented on Figure N39.

Fig. N38: Shape of the voltage supplied by a light dimmer at 50% of maximum voltage with the following techniques:
a] “cut-on control”
b] “cut-off control”

Fig. N39: Percentage of 3rd harmonic current as a function of the power applied to an incandescent lamp using an electronic dimmer switch

Note that in practice, the power applied to the lamp by a dimmer switch can only vary in the range between 15 and 85% of the maximum power of the lampere
According to IEC standard 61000-3-2 setting harmonic emission limits for electric or electronic systems with current ≤ 16 A, the following arrangements apply:

• Independent dimmers for incandescent lamps with a rated power less than or equal to 1 kW have no limits applied
• Otherwise, or for incandescent lighting equipment with built-in dimmer or dimmer built in an enclosure, the maximum permissible 3rd harmonic current is equal to 2.30 A

Fluorescent lamps with magnetic ballast

Fluorescent tubes and discharge lamps require the intensity of the arc to be limited, and this function is fulfilled by a choke (or magnetic ballast) placed in series with the bulb itself (see Fig. N40).

Fig. N40: Magnetic ballasts

This arrangement is most commonly used in domestic applications with a limited number of tubes. No particular constraint applies to the switches.
Dimmer switches are not compatible with magnetic ballasts: the cancellation of the voltage for a fraction of the period interrupts the discharge and totally extinguishes the lampere
The starter has a dual function: preheating the tube electrodes, and then generating an overvoltage to ignite the tube. This overvoltage is generated by the opening of a contact (controlled by a thermal switch) which interrupts the current circulating in the magnetic ballast.
During operation of the starter (approx. 1 s), the current drawn by the luminaire is approximately twice the nominal current.
Since the current drawn by the tube and ballast assembly is essentially inductive, the power factor is very low (on average between 0.4 and 0.5). In installations consisting of a large number of tubes, it is necessary to provide compensation to improve the power factor.
For large lighting installations, centralized compensation with capacitor banks is a possible solution, but more often this compensation is included at the level of each luminaire in a variety of different layouts (see Fig.N41).

Compensation layout
Without compensation Domestic  Single connection
Parallel [a] Offices, workshops, superstores Risk of overcurrents for control devices
Series [b] Choose capacitors with high
operating voltage (450 to 480 V)
Duo [c] Avoids flicker

Fig. N41: The various compensation layouts: a] parallel; b] series; c] dual series also called “duo” and their fields of application

The compensation capacitors are therefore sized so that the global power factor is greater than 0.85. In the most common case of parallel compensation, its capacity is on average 1 µF for 10 W of active power, for any type of lampere However, this compensation is incompatible with dimmer switches.

Constraints affecting compensation

The layout for parallel compensation creates constraints on ignition of the lampere Since the capacitor is initially discharged, switch-on produces an overcurrent. An overvoltage also appears, due to the oscillations in the circuit made up of the capacitor and the power supply inductance.
The following example can be used to determine the orders of magnitude.
Assuming an assembly of 50 fluorescent tubes of 36 W each:

• Total active power: 1,800 W
• Apparent power: 2 kVA
• Total rms current: 9 A
• Peak current: 13 A

With:

• A total capacity: C = 175 µF
• A line inductance (corresponding to a short-circuit current of 5 kA): L = 150 µH

The maximum peak current at switch-on equals:
$Ic = V_{max} \sqrt {\frac{c}{L} }= 230 \sqrt{2}\sqrt {\frac{175 \times 10^{-6} }{150 \times 10^{-6} } }=350 A$
The theoretical peak current at switch-on can therefore reach 27 times the peak current during normal operation.
The shape of the voltage and current at ignition is given in Figure N42 for switch closing at the line supply voltage peak.

Fig. N42: Power supply voltage at switch-on and inrush current

There is therefore a risk of contact welding in electromechanical control devices (remote-control switch, contactor, circuit-breaker) or destruction of solid state switches with semi-conductors.
In reality, the constraints are usually less severe, due to the impedance of the cables.
Ignition of fluorescent tubes in groups implies one specific constraint. When a group of tubes is already switched on, the compensation capacitors in these tubes which are already energized participate in the inrush current at the moment of ignition of a second group of tubes: they “amplify” the current peak in the control switch at the moment of ignition of the second group.
The table in Figure N43, resulting from measurements, specifies the magnitude of the first current peak, for different values of prospective short-circuit current Isc. It is seen that the current peak can be multiplied by 2 or 3, depending on the number of tubes already in use at the moment of connection of the last group of tubes.

Number of tubes already in use
Number of tubes
connected
Inrush current peak (A)
Isc = 1,500 A  Isc = 3,000 A Isc = 6,000 A
0 14 233 250 320
14 14 558 556 575
28 14 608 607 624
42 14 618 616 632

Fig. N43: Magnitude of the current peak in the control switch of the moment of ignition of a second group of tubes

Nonetheless, sequential ignition of each group of tubes is recommended so as to reduce the current peak in the main switch.
The most recent magnetic ballasts are known as “low-loss”. The magnetic circuit has been optimized, but the operating principle remains the same. This new generation of ballasts is coming into widespread use, under the influence of new regulations (European Directive, Energy Policy Act - USA).
In these conditions, the use of electronic ballasts is likely to increase, to the detriment of magnetic ballasts.

Fluorescent lamps with electronic ballast

Electronic ballasts are used as a replacement for magnetic ballasts to supply power to fluorescent tubes (including compact fluorescent lamps) and discharge lamps. They also provide the “starter” function and do not need any compensation capacity.
The principle of the electronic ballast (see Fig. N44) consists of supplying the lamp arc via an electronic device that generates a rectangular form AC voltage with a frequency between 20 and 60 kHz.

Fig. N44: Electronic ballast

Supplying the arc with a high-frequency voltage can totally eliminate the flicker phenomenon and strobe effects. The electronic ballast is totally silent.
During the preheating period of a discharge lamp, this ballast supplies the lamp with increasing voltage, imposing an almost constant current. In steady state, it regulates the voltage applied to the lamp independently of any fluctuations in the line voltage.
Since the arc is supplied in optimum voltage conditions, this results in energy savings of 5 to 10% and increased lamp service life. Moreover, the efficiency of the electronic ballast can exceed 93%, whereas the average efficiency of a magnetic device is only 85%.
The power factor is high (> 0.9).
The electronic ballast is also used to provide the light dimming function. Varying the frequency in fact varies the current magnitude in the arc and hence the luminous intensity.

Inrush current

The main constraint that electronic ballasts bring to line supplies is the high inrush current on switch-on linked to the initial load of the smoothing capacitors (see Fig. N45).

Technology Max. inrush current Duration
Rectifier with PFC 30 to 100 In ≤ 1 ms
Rectifier with choke 10 to 30 In ≤ 5 ms
Magnetic ballast ≤ 13 In 5 to 10 ms

Fig. N45: Orders of magnitude of the inrush current maximum values, depending on the technologies used

In reality, due to the wiring impedances, the inrush currents for an assembly of lamps is much lower than these values, in the order of 5 to 10 In for less than 5 ms.Unlike magnetic ballasts, this inrush current is not accompanied by an overvoltage.

Harmonic currents

For ballasts associated with high-power discharge lamps, the current drawn from the line supply has a low total harmonic distortion (< 20% in general and < 10% for the most sophisticated devices). Conversely, devices associated with low-power lamps, in particular compact fluorescent lamps, draw a very distorted current (see Fig. N46). The total harmonic distortion can be as high as 150%. In these conditions, the rms current drawn from the line supply equals 1.8 times the current corresponding to the lamp active power, which corresponds to a power factor of 0.55.

Fig. N46: Shape of the current drawn by a compact fluorescent lamp

In order to balance the load between the different phases, lighting circuits are usually connected between phases and neutral in a balanced way. In these conditions, the high level of third harmonic and harmonics that are multiple of 3 can cause an overload of the neutral conductor. The least favorable situation leads to a neutral current which may reach $\sqrt 3$ times the current in each phase.
Harmonic emission limits for electric or electronic systems are set by IEC standard 61000-3-2. For simplification, the limits for lighting equipment are given here only for harmonic orders 3 and 5 which are the most relevant (see Fig. N47).

Harmonic order Active input power > 25W Active input power  ≤ 25W one of the 2 sets of limits apply:
% of fundamental current  % of fundamental current Harmonic current relative to active power
3 30 86 3.4 mA/W
5 10 61 1.9 mA/W

Fig. N47: Maximum permissible harmonic current

Leakage currents

Electronic ballasts usually have capacitors placed between the power supply conductors and the earth. These interference-suppressing capacitors are responsible for the circulation of a permanent leakage current in the order of 0.5 to 1 mA per ballast. This therefore results in a limit being placed on the number of ballasts that can be supplied by a Residual Current Differential Safety Device (RCD).
At switch-on, the initial load of these capacitors can also cause the circulation of a current peak whose magnitude can reach several amps for 10 µs. This current peak may cause unwanted tripping of unsuitable devices.

High-frequency emissions

Electronic ballasts are responsible for high-frequency conducted and radiated emissions.
The very steep rising edges applied to the ballast output conductors cause current pulses circulating in the stray capacities to earth. As a result, stray currents circulate in the earth conductor and the power supply conductors. Due to the high frequency of these currents, there is also electromagnetic radiation. To limit these HF emissions, the lamp should be placed in the immediate proximity of the ballast, thus reducing the length of the most strongly radiating conductors.

The different power supply modes (see Fig. N48)

Technology Power supply mode  Other device
Standard incandescent  Direct power supply Dimmer switch
Halogen incandescent
ELV halogen incandescent Transformer Electronic converter
Fluorescent tube Magnetic ballast and starter Electronic ballast
Electronic dimmer + ballast
Compact fluorescent lamp Built-in electronic ballast
Mercury vapor Magnetic ballast Electronic ballast
High-pressure sodium
Low-pressure sodium
Metal halide

Fig. N48: Different power supply modes