The DPI's position on compliance with the standards is clearly spelled out by John Waudby.
"Most Standards are not prescribed in legislation, however, the latest coal legislation includes objectives for electrical and mechanical engineering safety that require safeguards to have a suitable safety integrity (the words used are actually probability of failure), from this our position is the use of AS61508, AS62061 and AS4024," he said.
Mr Waudby also said the DPI was planning safety seminars to promote the adoption of the standards and noted that: "The DPI is represented on Standards committees for AS4024 and AS62061 and actively pursues integration of these standards in mining equipment specific standards and DPI guidelines and technical references."
The DPI refers to three standards:
1. AS 4024.1-2006 Series: Safety of machinery
For a guide to making metalworking machinery safe, look no further than Australian Standard AS 4024.1. The standard was extensively updated and expanded just last year but the basic principles remain. It shows that injuries are far from inevitable and some simple planning – beginning with a risk assessment followed by the risk control principles defined in the standard -- can dramatically reduce the likelihood of someone being hurt.
While people often refer simply to "risk assessment", this is just part of a more comprehensive and logical three-step process required under state laws:
1. Hazard identification
2. Risk assessment
3. Risk control
Put simply, hazard identification involves exploring Murphy's Law: what can go wrong, will go wrong. Listing every possibility is not always easy, so Pilz recommends having workers who regularly use and modify the equipment plus those who may be unfamiliar with it (such as OH&S staff & managers) involved in the process.
Once you have listed all the hazards, rate the potential injuries in terms of their severity and probability. AS4024.1 recommends using one of two codes: S1 or S2. If slight or reversible, such as a scratch or a bruise, it is classified S1, while S2 denotes serious or irreversible injuries, including amputation, deep wounds or death.
Next, consider the frequency of exposure and/or exposure time with the hazard in the same way. Low frequency, such as during maintenance only, is rated "F1". Frequent exposure (generally more than once per day/shift) would be considered "F2".
Finally, evaluate the possibility of avoiding the hazard. This is generally related to the speed of the hazard, the worker's proximity to the hazard, level of training, and expertise. If, in your opinion, the operator could recognize the hazard and avoid injury, select P1. Otherwise, select P2.
Once you have assigned a severity, frequency and possibility of avoidance level to each hazard, follow the matrix supplied in AS4024.1 to determine the category.
2. AS 61508.0-2006 : Functional safety of electrical/electronic/programmable electronic safety-related systems
On the surface, AS61508 and AS4024 have quite a bit in common. Both begin with a hazard identification or analysis process, both assess the level of risk involved and both assign a “safety integrity level” or “category” to define various levels of safety performance, but that is where the similarities end.
Perhaps the biggest difference between the two standards is the determination of the safety control system. Unlike the simple decision tree used by AS 4024.1, AS61508 asks for a quantitative measure of the overall failure rate of the safety system. The mean time between failures needs to be ascertained for each element of the system and then a cumulative probability of dangerous failure calculated.
Probability of plant failure to danger on demand means how often the plant is required to operate, the reliability of the system and the ability of the system to detect failures before a dangerous condition can result (or its diagnostic coverage). As with AS4024, the more serious the consequences of failure, the higher the SIL, and with an increased SIL ranking comes more stringent safety system requirements.
The standard’s key criteria for rating safety systems are strength, diagnostics capability, common cause strength and redundancy.
3. AS 62061-2006 : Safety of machinery - Functional safety of safety-related electrical, electronic and programmable electronic control systems
AS 62061 is a daughter standard of AS 61508 that deals specifically with the safety of machinery and follows the same format.
The SILs are roughly equivalent to the categories of AS 4024. For instance, a rigorous application of the principles of AS 4024 to category 4 will have an equivalence to SIL 3, AS 62061, in terms of the probability of a dangerous failure per hour (in this case, better than 10-7 ). AS 62061 is primarily aimed at the manufacturers and users of programmable safety systems, safe bus systems and safe software when a SIL is specified by the end user.
From a practical perspective, the technology needed to prevent UMOs is well developed and simple to implement. Failure of a contactor in the 'on' position is a classic case for dual redundancy monitored systems that detect faults and do not allow a single fault to cause the loss of safety.
A doubly-redundant safety control system such as the Pilz Programmable Safety System (PSS) used on the shuttle car and the continuous miners would have detected single faults and prevented loss of safety requiring the fault to be rectified before the machinery could restart. Further, it would require reset and then another separate start command before the machinery could move.
At Pilz, we always recommend adopting the Australian Standards because that is both good business practice and the right way to approach safety. You will certainly minimise the legal liability of managers and employers but, more importantly, you might well save a workmate's life by avoiding incidents in the first place.
