End-of-life effect of explosion-protected luminaires with fluorescent lamps intended for zone 1

Deutsche Version

Background

Explosion-protected luminaires with fluorescent lamps, designed to Increased Safety as the dominating type of protection, have established themselves in the market as an economical and reliable general-purpose lighting means for potentially explosive atmospheres. For Germany alone it is assumed that more than 1 million luminaires of this type have been installed in different versions, and that the production volume per year accounts for more than 300,000 units.

Up-to-date technical standards require T8 fluorescent lamps (dia. 26 mm) with G13 caps to be operated with short-circuited cap pins to safeguard the Increased Safety type of protection. This means that the filaments cannot be heated and monitored. The available standards primarily specify external parameters for these types of lamps, without giving any consideration to such specific physical properties as the emitter material, etc.

In the more recent past, luminaires used in conjunction with these fluorescent lamps have been found to fail as a result of local overheating of the lamp cap and the lamp socket. There are different kinds of faults that may have led to these failures. One possible explanation is the end-of-life effect (EOL) of the lamp, which will occur only in exceptional cases at the end of the lamp lifetime. It is for the time being not possible to reproduce this EOL effect in the laboratory in a conclusive manner, but it may be described as follows in qualitatively terms.

When the emitter material of the cathode (the filament of the conventional bi-pin lamp) has been used up, or when it loses its emissivity for other reasons, the emission of electrons will be impeded. As a result, the cathode voltage drop will increase considerably. Frequent starting from the cold state tends to accelerate this loss of active emitter material. Operation of the lamp on constant current (the electronic ballast can by approximation be compared to a constant-current source) will go along with a high power loss, which may also heat up, and possibly damage, the lamp cap and the lamp socket. This process is often referred to as the EOL effect. In electrotechnical terms it becomes effective in the form of the “partial rectifier effect”. The surface temperatures temporarily occurring at the lamp ends, clearly exceed the limits specified for explosion protection. They may damage elements like the lamp socket to such an extent that their explosion protection function will no longer be safeguarded.

This effect may in a similar way also occur in connection with single-pin lamps, which means that a flameproof lamp socket would not provide any additional safety either.

Measures as part of the quality requirements

Since explosion-protected equipment has to comply with product-specific standards, in addition to the specific explosion protection standards, the DIN EN 61347-2-3 (VDE 0712 - 33) standard that came into effect in February 2005 for AC-current-operated electronic ballast used in connection with fluorescent lamps has to be considered in addition. This standard calls for a permanently effective EOL safety shutdown, whose function may be tested by means of three different test circuits (asymmetric pulse test, asymmetric power test, or open filament test). Once one of these three tests has been passed, the ballast is assumed to provide adequate protection for universal purpose lighting with respect to the EOL behaviour of fluorescent lamps. However, for the time being this only applies to T5 and T4 lamps (16 mm diameter or thinner) in order to avoid excessively high temperatures for fire danger purposes.

For explosion-protected luminaires designed to Increased Safety type of protection, other considerations come into play.

Aspects to be considered include that

1. lamps with bi-pin caps used for Ex-e luminaires have their filaments short-circuited,
2. normally T8 lamps with 26-mm diameter are used, and
3. lamps with single-pin caps may also be used.

On the basis of an extensive test programme conducted by the PTB with the support of the suppliers of luminaires and lamps and the German ZVEI, a proposal for the standard IEC 60079-7 "Increased Safety" has been made. The proposal was transferred to the maintenance team (MT 60079-7) in IEC TC31 and was published (in a slightly modified wording) in IEC 60079-7:2006. Following the relevant part of IEC 60079-7:2006 (will be published also as EN 60079-7 early in 2007):

5.3.7 Luminaires for tubular fluorescent bi-pin lamps

5.3.7.3 Temperature class
As the limiting temperature of a luminaire with tubular fluorescent bi-pin lamps employing an electronic ballast will exceed the temperatures appropriate for temperature classes T5 and T6, those temperature classes shall not be permitted. See 6.3.2.3.

6.3.2.3 Power dissipation of cathodes of lamps supplied by electronic ballasts
The asymmetric pulse test and the asymmetric power dissipation test according to Annex H shall be conducted. For T8, T10, and T12 lamps, the maximum cathode power observed during the tests shall not exceed 10 W.
The values for the maximum cathode power of size T4 (12 mm) and T5 (16 mm) lamps in increased safety luminaires is under consideration.
NOTE The limits for the power dissipation in cathodes of lamps supplied by electronic ballasts limits were derived from experimental data on luminaires operated in an ambient temperature of 60 °C and with a temperature class of T4.

Even if the 10-W limit is complied with, a diethyl ether/air mixture as a test mixture for T4 may under laboratory conditions ignite in very rare exceptional cases. In view of the very low likelihood of its occurrence, this situation can be accepted under safety aspects, because Directive 94/9/EC, annex II, paragraph 2.2.1.1, requires the following for category-2 equipment (use in zone 1):

“Equipment must be so designed and constructed as to prevent ignition sources arising, even in the event of frequently occurring disturbances or equipment operating faults, which normally have to be taken into account.”

The proposal made for translation of these results into the practical certification work has been discussed and confirmed among the European notified bodies (ExNB). January 1, 2005 is the date at which the above requirements of the draft standard (IEC 60079-7, 6.3.2.2) become effective for applications filed for the testing and certification of electronic ballast.

Measures as part of the operating instructions

The measures mentioned are intended for newly issued certificates. This is to allow any insights gained to be used for technical improvements.

The assessment of potential ignition risks, and the number of faults that have been identified to the present day, do not justify the requirement of having previously certified luminaires of category 2G (zone 1) and category 3G (zone 2) retrofitted or converted.

It may, however, be useful check luminaires for any changes as part of preventive maintenance measures. Tube ends that have turned dark, or unusual luminaire lighting behaviour might be indicative of the EOL effect. To be on the safe side, the luminous element concerned should in that case be replaced.