Barrier protects taeho from hazari
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PETROL tankers and vehicles carrying dangerous goods must be fitted with a battery master switch to isolate the complete electrical system when the vehicle is standing in a hazardous area. In countries where tachographs are a legal requirement, the regulations state that the clock must be driven continuously whether the vehicle is moving or not — and the clock is electrically driven. Problem.
To meet these two conflicting requirements has caused the industry to invest in a lot of research to come up with an answer. I have been speaking to Lucas Kienzle to find out the ins and outs of having an intrinsically safe tachograph.
it is worth defining a few of the legal requirements at this point. Such vehicles involved in the carriage of dangerous goods as defined under the Explosives Act of 1875 and the Petroleum (Consolidation) Act of 1928 must be fitted with a master switch in order to minimise dangers from an electrical discharge when the vehicle is standing in a hazardous area.
The definition of "hazar dous area" is not as simple as it sounds being sub-divided by the International Electrotechnical Commission (known as the IEC) into three separate categories or "zones:" • Zone 0, where an explosive air/gas mixture is present continuously or present for long periods. A good example of this is the inside of a storage tank. • Zone I, where an explosive mixture is likely to occur in normal operation.
• Zone 2 is when such a mixture is not likely to occur but if it does it will only exist for a short term.
The problem with the master switch arises because, with the original definition of the requirements, such an arrangement would stop every single electrical function on the vehicle, in cluding the time clock of th tachograph — which wouf result in an incomplete cite, record. It would also meat that the clock would neei resetting after every elec trice! shut-down.
The start of this contradic tion goes back to the switch over from a mechanical clod to an electronic quartz clod around 1974. Although at thi time it was suggested the tachographs should go badl to mechanical clocks, at leas as far as Pet Reg vehicle; were concerned, the opera tors were not keen as the thought their drivers woult be at a disadvantage havini to remember to wind up tht clock.
This type of electrical pro. blem is not new to the oil and mining industries and an en. gineering discipline has been developed to enable elec. tricity to be used in areas where an explosive atmosphere may exist at any time. Intrinsically safe protection is defined at the Safety in Mines Research Establishment (SMRE) at
leffield with approval and Irtification being dealt with F the British Approvals Serce for Electrical Equipment
Flammable Atmospheres ■ ASEEFA) at Buxton.
The basic requirement is at the energy available om the power source must insufficient to ignite the is/air mixture. Relating this our particular case of the Aker, this means that when le battery master switch is perated, any current which allowed to by-pass the Nitch has to be of such a ature that it cannot cause an (plosion.
A "standard" explosive lixture is used for tests, onsisting of 80 per cent air rid 20 per cent hydrogen, as lie is reckoned to be about le worst case likely to be ncountered. As well as the 'hole circuit being intrinsially safe, each individual omponent must be as well, o that there is no danger of n accidental short circuit or arth within the component..
But it doesn't stop there. Jot only must the circuit be Itrinsically safe when it is in lerfect condition, it must ilso meet the requirements yen with two earth faults lxisting within the circuit!
Lucas Kienzle Instruments .td has obtained full 1ASEEFA certification for in intrinsically safe tachopraph system for dangerous poods vehicles which require battery master switch. The lystem incorporates a barrier device to limit the current by-passing the battery master switch, a revised electronic circuit within the clock mechanism and a modification to the CAV 444 master switch.
The revised electronic circuit was introduced so that the capacitors inherent in electronic clock operation do not feed power back into the wiring circuit. With the tachograph clock, the 10 microfarad capacitor contained enough charge to ignite an explosive mixture when the master switch was operated.
Lucas Kienzle looked at the possibility of changing the capacitor, but they couldn't get one small enough which would still fulfil the required function. The alternative was to encapsulate the whole printed circuit so, if the capacitor did discharge, the spark would be contained. The encapsulation is by means of a non-porous epoxy resin "gunge" which is sprayed over the assembly.
Still with the printed circuit, no two conductors are allowed to be situated closer than 3mm apart so the circuit had to be redesigned to meet this requirement. The CAV 444 switch has been modified and it now bears a label on the side of the base moulding with the designation CAV 444 Ex /BAS No. 2108U.
Now to the barrier device. The circuit inside the tachograph is fairly complex in that there are two separate positive supplies being put in — one is simply for panel il lumination and the other drives the clock. The panel lights also include the tachograph warning light which warns the driver of any malfunction such as a sticking stylus or the absence of a chart.
Lucas Kienzle uses a warning light rather than a window on to the chart as the regulations state that "the equipment shall be so designed that it is possible without opening the case to check that recordings are being made”. Note the use of the word "check" which some tachograph manufacturers have interpreted as "see."
With two positive supplies being put in, there are two negatives coming out. One terminal copes with everything bar the clock, which is dealt with by a second terminal. The barrier unit allows a current to bypass the master switch but in such a way that the current is limited and is supplied only to the clock circuit. It allows 60mA to flow under normal operating conditions and a maximum of 120mA under the BASEEFA "two fault" condition.
The barrier unit is mounted on the master switch itself as the further away it is moved, the more "unsafe" wire is present.
As a quick check on the safety of a tanker electrical system Lucas Kienzle suggests a three point check: • Check the tachograph to see if it is intrinsically safe. If it meets with BASEEFA approval it has a small crown insignia on the tacho itself complete with the lettering • Check that the battery cutoff switch has been modified.
• Check that the barrier device has been installed correctly at the master switch.
It is a sobering thought that an explosion within a chemical/petroleum plant can cause great loss of life and around £.15 million of damage. It pays to be sure.
by Graham Mon tgomerie