Philips Electromagnetic Lamp 3 18Circuit breakers, fusing and earth leakage, Standard conditions

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3.17 Stroboscopic effect and striations

depends on several factors, including lamp position, supply voltage, temperature, age of the lamp (electrode) and also of the lamp current wave form (peak factor).

4. Striations are noticeable as a pattern of more or less bright regions in the long discharge tube of fluorescent lamps.The pattern can move through the discharge tube. It can appear when the lamp is cold or when the lamp is dimmed down to too low a level.

3 18 Circuit breakers, fusing and earth leakage

1 Standard conditions

Under normal conditions the highest current which can occur is the current during the starting phase.When the starter is closed, practically the entire supply voltage is across the ballast, resulting in a high current and a low power factor.The fuses must be capable of handling this high initial current for several minutes. For most of the fluorescent lamps stabilised with copper/iron ballasts, this starting current is about 1.5 times the normal operating current.

During switching on, a few other processes are going on as well:

- the (empty) parallel compensating capacitor will be charged with a high inrush current,

- depending on the magnetic saturation of the ballast a voltage induction will take place in the ballast,

- gas-discharge lamps can have some rectification or DC component in the lamp current.

These phenomena occur in the very first 3 to 5 milliseconds and can result in a peak current of 15 to 25 times the nominal current.This surge current will depend on the lamp and ballast type and the number of lamps per circuit as well as, of course, on the resistance and impedance of the lamp and supply cables and the impedance of the mains supply network.This latter part varies greatly in practice. It is recommended that a surge current of 20 to 25 times the nominal current during the first 3 milliseconds be used and 7 times the nominal current for the first 2 seconds for parallel compensated circuits as a guide for selecting fuse ratings.

In the duo-circuit the capacitor is connected in series with the coil, so the very high surge currents cannot appear in this case.

Devices for switching and fusing must be capable of handling these currents correctly.This means that for fuses slow-acting gI types (normal general purpose type for cable fusing) have to be used (German name:gL). The main purpose of the fuse is to protect the cable and the distribution part of the lighting installation from damage in case of a failure in the installation. So the fuse rating is primarily related to the cable core used in the installation.

As the various national electrical safety rules differ slightly, the recommended fuse ratings for lighting equipment published by the various lamp, gear and fuse suppliers are not always the same. Moreover, there are differences in the various brands of fuses.

As a guide, it is recommended to load gl-fuses to not more than 50 - 70 per cent of their rating.

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