Summary

This study was initiated by an October 25, 2013 letter written by a Union member of the sub-committee on hoisting machines of the Commission de la santé et de la sécurité du travail (CSST)¹. The letter asked the sub-committee to mandate the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) to evaluate emergency arrest systems (safety catches and other systems) for mine conveyances in use worldwide, ultimately with a view to modernizing the safety catches required on mine conveyances in Québec. Conveyance crashes result from two different hazardous events: (i) hoist rope failure and (ii) loss of control of cage movement with no rope failure. This report, which covers Part 1 of the above mandate, provides a state of the art on mine conveyance accidents, hoist ropes, safety catches, safety systems for preventing loss of control of cage movement, and the different regulations governing hoist ropes and safety catches across Canada. Parts 2 and 3 of this mandate will examine the two aforementioned types of hazardous events.

Most of the underground mine shafts in Canada have drum hoists equipped with a single hoist rope attached to the conveyance. A safety catch system is mandatory to prevent cage crashes in the event of rope failure. This system was invented in the mid-19th century by the Frenchman Fontaine, and a similar system was patented by Otis a few years later for civilian elevators. The current system is the result of improvements made following the 1945 Paymaster (Ontario) accident, which claimed 16 lives. Safety catches are still required today under most North American regulations, but are no longer used in Europe or South Africa.

Rope failure incidents are relatively rare, and none occurred in Ontario between 1994 and 2004. Analysis of three recent accidents in Québec (2009, 2011, 2013) showed that rope failure was not the cause. Also in Québec, rope failure has been the immediate and direct cause of only one cage or skip accident in the past 30 years. However, a handful of cases (5) involving rope failure in Canada in the past 20 years were mentioned by the individuals we interviewed. In the United States, mine conveyance accidents or incidents are also relatively rare (0.41% of mining fatalities). Documented cage crashes fall into two categories: vertical shaft and cage crashes into the headframe (friction hoist); inclined shaft and cage crashes at the bottom of the shaft following a failure in the braking system.

The fault tree (FT) method is used to understand the root causes of conveyance crashes at the bottom of the shaft. Two FTs are presented in this report. The first concerns cage crashes at the bottom of the shaft (undesired event) following rope failure. The second concerns cage crashes at one of the ends of the shaft (undesired event) with no rope failure. These fault trees will be detailed in parts 2 and 3 of the mandate respectively and will serve to identify the strengths and weaknesses of the different safety systems.

There are two main schools of thought regarding mine safety: (i) the best safety system is to use a good-quality hoist rope that is well maintained and replaced regularly (South Africa), and (ii) safety catches are needed to prevent cage crashes because hoist ropes break despite the progress made and inspections (North America). Hoist ropes are susceptible to wear and must be monitored and changed regularly, unlike hoisting machines, which are generally used for the entire operating life of the shaft. Hoist ropes are selected on the basis of their use and have varying life spans depending on use and the equipment involved: from 12 to 18 months for drum hoists when shafts are very deep (up to 3 or 4 years for shallower shafts), and from 24 to 48 months for friction hoists (up to 7 years for shallower shafts). Mixed-material (steel/synthetic) hoist ropes are gradually being introduced in shafts, and in the longer term, conceivably 100% synthetic hoist ropes will be used. Hoist ropes are subject to wear by abrasion, corrosion (external and/or internal) and fatigue (rope failure over time). Care must also be taken to limit dynamic loads (sudden acceleration or braking, defects in the shaft guides, lack of roundness of the sheave, etc.), as well as slackening of tension in the hoist rope, which can lead to failures. Daily visual inspections are performed to ensure that the rope can be used risk-free. These inspections are limited to the outer, visible part of the hoist rope, and it must be very clean for optimal inspection conditions. Break tests are carried out at regular intervals to assess the state of wear of the rope, and non-destructive electromagnetic tests are also conducted. However, break tests are performed on a section of the hoist rope close to the conveyance attachment to limit the shortening of the rope. This test may, therefore, not be representative of the actual breaking strength of the rope over its entire length, since the section tested is probably the part of the rope subject to the least stress (no friction, no passage on the sheave, and lowest load). However, this part of the rope is also prone to corrosion. Devices for continuous monitoring of hoist ropes are now appearing on the market, and include two types of systems: electromagnetic systems and optical systems. When combined with a comprehensive rope monitoring program, these systems reduce the safety factor required of the hoist rope.

Safety catches are designed to stop falling cages in the event of hoist rope failure. The traditional system, found on all cages in Québec, is the result of studies conducted following the Paymaster accident. It requires wooden shaft guides and is triggered when there is insufficient tension on the hoist rope. Its operating principle – braking through wood chipping – provides little control over deceleration. In addition, the condition of the wooden shaft guides and the relative position of the guides/teeth influence the deceleration. Lastly, an empty cage means greater deceleration. Modern systems operating on metal guides have been proposed by two companies in Canada, but to date, very few mines are equipped with them. The metal-guide positioning systems of these companies are evolving. One of these systems underwent free-fall testing (empty cage) and an average deceleration of 1.44 g was measured. Fully loaded cage testing is expected to be carried out in South Africa in the near future. One of the undesirable drawbacks of these systems is the risk of unintentional activation.

Loss of control of the cage’s movement was the hazardous event that led to most of the recent accidents documented in this report. This aspect will be detailed in Part 3. Several redundant braking systems (mechanical brake on the drum, dynamic brake, and possibly a hoist rope brake) can be used to prevent loss of control of the cage’s movement. Apart from these different braking systems, speed controllers are used to prevent loss of control of the cage’s movement. Traditionally, the Lilly controller is used, but it is increasingly being replaced by digital controllers. The RF-412 data sheet published by the IRSST in 2005 and cited in section 216.1 of Québec’s Regulation respecting occupational health and safety in mines (ROHSM) deals with programmable control systems².

However, it recommends keeping an independent electromechanical safety system. Lastly, some recent studies are leaning toward comprehensive monitoring of all components of the hoisting system to ensure mine cage safety.

The last part of this report compares provincial regulations governing hoist ropes and safety catches. Safety catches are mandatory in all Canadian provinces for cages supported by a single hoist rope. All provinces also require daily inspection, which is solely visual in some provinces but more or less comprehensive in others. Quick-release tests must be performed at regular intervals (null initial speed) to ensure proper functioning of the system. A free-fall test (initial speed differs from zero) is mandatory before putting the cage into service and after making any changes to the safety catches or the cage. Most of the provinces prescribe an acceptable deceleration interval for free-fall tests of between 1 g and 3 g or between 0.9 g and 2 g, depending on the province (there is no deceleration interval prescribed in Québec’s ROHSM).

The safety factors (SFs) for hoist ropes are relatively similar between provinces. In general, an SF of 5.0 on the head sheave is needed. Two provinces (Ontario and Québec) allow for reducing this SF, provided that the inspection methods used in South Africa are applied (and in Québec, that a program of continuous monitoring of the condition of the hoist rope is implemented). Hoist ropes must be lubricated regularly and daily inspection is mandatory (in Québec, this inspection may be replaced by continuous monitoring of the state of wear of the hoist rope or by an electromagnetic inspection, as per section 305 of the ROHSM). Two other types of tests are mandatory in all the provinces: non-destructive tests and break tests. The frequency of these tests varies significantly from province to province. Break tests are performed on samples measuring 2.5 m in length, and it appears that the only laboratory authorized to conduct such tests is that of the Ontario Ministry of Labour in Sudbury. A few provinces refer to CSA Standard CAN/CSA G4 for testing modalities, but in actual fact, all break tests appear to be conducted according to this standard. Lastly, the discard criteria for hoist ropes are relatively similar: loss of resistance of less than 10%, number of broken wires less than 5% on one lay length (main cause of discard, in fact), reduced elasticity of the hoist rope. Certain provinces add other discard criteria. Standard ISO 4309 (not referenced in the different provincial regulations) proposes a method for calculating the cumulative effects of wear and tear on hoist ropes.

[1] Now the Commission des normes, de l'équité, de la santé et de la sécurité du travail (CNESST).

[2] The RF-412 data sheet has been updated, and the new version, RF-1049, was published in 2019.