SATRA’s Peter Doughty talks about the hazards associated with live electrical working and the tests that are carried out on gloves used to offer protection against high voltages.
Gloves can be designed to provide many benefits for the wearer. Common examples of this are:
• Thermal insulation – either in terms of keeping the hands warm in a cold environment or protecting against heat, perhaps when handling hot components. There are also specialised gloves for welding, foundry work and for firefighters
• Mechanical protection – such as prevention of cuts or lacerations in either a work environment or during leisure activities such as cycling. This category of glove would also include chainmail gloves as used in abattoirs, or forestry gloves used when operating chainsaws
• Chemical protection – which would include at one extreme simple household dishwashing gloves right through to heavy duty gloves used when handling industrial chemicals
• Microbiological protection – which tend to be disposable products used by the medical profession – everyone from dentists through to surgeons
A simple visual inspection of the above gloves can often give a reasonable indication of the level of protection likely to be afforded. One property that can never reliably be assessed visually, however, is the electrical resistance between the inner and outer surfaces of a glove – sometimes referred to as vertical resistance, rather than surface resistance which, as the name suggests, is a measure of the electrical resistance across one surface of the glove. This property may not immediately come to mind, or even seem to be particularly important, but it is relevant to a wide range of specialist end use applications. Whether the resistance should be high or low is clearly dependent on why and where the glove is to be worn. If the glove is being worn in an environment where a build up of static electricity is to be avoided, such as when working with static sensitive products, or in a potentially explosive atmosphere, then the vertical resistance should be low. This, when used with other equipment such as antistatic wrist straps or footwear, will ensure that any static charge is safely discharged without causing damage.
When working on live electrical equipment with exposed parts that may be electrically live, however, it is important that the wearer does not introduce a conductive path – either through the body and footwear to ground, or from one hand to another part of the body (such as the other hand) that may be touching another part of the equipment that is at a different potential. An artificially generated flow of electric current through the human body may produce injury or death by affecting muscles and nerves, initiating abnormal electrical rhythms in the heart and brain, or producing internal and external electrical burns. Alternating current (AC) may produce ventricular fibrillation of the heart if the path of the current involves a passage through the chest cavity. This may occur when the current flows from arm to arm, arm to leg, or head to arm or leg. Charts are available that show the probability of fibrillation with respect to magnitude of the electric current and the time for which it is present. Even voltages as low as 50 V are capable of producing heart fibrillation, if they produce electric currents in body tissues which happen to pass through the chest area. The risk of electrocution depends on the conductivity of the area of human skin in contact with the voltage source. If skin is wet, or if there are wounds, or if the live conductor penetrates the outer skin layers, then even voltage sources below 40 V can be highly dangerous if contacted. The exact effects of electricity flowing through the human body vary from a tingling sensation at a current of 1-2 mA, through pain at 5 – 6 mA, to muscle spasms above 20 mA, and as the voltages increase so do the risks of serious injury. As people we are all different and the above current threshold values are only approximate, but the dangers are clear. UK statistics (as published by the Electrical Safety Council) show that while many people receive mains voltage electric shocks every year – estimated at around 2.5 million – 350,000 of these received a serious injury, 28 of which were fatal. Other data from the USA (Bureau of Labor Statistics and Electrical Safety Foundation International) suggests that in the five year period between 2003 and 2007 there were 1,213 fatal workplace accidents caused by contact with electricity, making this the seventh leading cause of occupational fatality.
As with all tasks, the hierarchy of risk reduction always starts with trying to do the work in a different way that either removes or reduces the hazard to an acceptable level. For instance, the risk of shock can be removed if it is possible to work on the equipment with the supply disconnected, locked off and earthed. If the equipment cannot be made safe or the job done in a different way, however, the use of Personal Protective Equipment (PPE), such as protective gloves which have been tested and approved as providing an acceptable level of protection against contact with a high voltages, will be required. In order to prevent dangerous levels of current through the wearer’s body, a layer of electrically insulating material needs to be introduced to break any possible circuit. A number of items of PPE are available to provide protection against electric shock, so in addition to products such as gloves and mitts used in the electrical industry for live working applications, there are helmets such as those used by firefighters, and sleeves. One regularly used protective mechanism against such electrical hazards is footwear with a very high electrical resistance. This is required to protect the wearer when there is the possibility of a large potential difference (voltage) between the wearer’s hand or body and the ground that he or she is standing on. Insulating footwear is available that has good insulating properties when subjected to high voltages. While standard gloves produced from rubber or polymeric compounds not containing any conductive impurities may offer some degree of protection against low voltages, specially designed gloves are necessary to ensure reliable protection in such applications. Higher voltages of, say, up to 1,000 V, while still generally referred to in the electrical industry as low voltages, require gloves which meet specialised designs and testing requirements. The most common type of product is likely to be an all-rubber or all-polymeric glove. Test methods for products usually involve measuring leakage currents through the full thickness of the glove’s construction when high voltages, such as up to 50,000 V, are applied between the inner and outer surfaces.
Testing for safety
There are a number of national and international standards designed to assess insulation resistance and all generally use a similar principal of test. Typically these include tests at relatively high voltages – often referred to as a Proof Voltage Tests. For these tests electrodes or probes are applied to the inside and outside of the glove and a gradually increasing voltage applied until the limit specified is reached. This level of voltage is then held for a defined period usually between one and several minutes, depending on the product standard, while the leakage current is monitored. Then, some standards such as those used for insulating gloves include a second test to assess dielectric strength or electrical flashover, often referred to as a withstand voltage test. These tests are similar in principle to the Proof Voltage Test, except that a higher voltage is applied, usually around twice the voltage used in the proof test; this voltage does generally not need to be held for more than a few seconds and there are normally no performance requirements for leakage current, just that the glove material should not break down or puncture. Testing can be carried out using either an AC (alternating current) or DC (direct current) source. A puncture is, as the name suggests, a small hole created in the glove by the flow of electric current due to a breakdown in insulating properties. Often the electrical test equipment incorporates a short circuit current safety trip so if ‘flashover’ e.g. insulation breakdown occurs, the trip will operate. Many test specifications, such as those applied to gloves, include a wet test to ensure that all parts of the item meet the minimum required performance characteristics. For this, the glove is placed in a tank and both the sample and the tank are filled with water. Care is taken to exclude any air pockets or bubbles, which therefore ensures continuous contact inside and outside the product under test. One probe is placed inside the glove and the other outside in the tank, and the test voltage applied in the normal way. Similar test methodology is applied to insulating footwear.
Glove testing in Europe
The European standard covering such gloves is EN 60903: 2003 ‘Live Working – Gloves of insulating material’, which is derived from IEC 60903: 2003. This includes a range of tests, some of which are optional and only carried out if a specific property is to be claimed (see Table 2). The mandatory test assessments include checks on dimensions, finish, marking and packaging, plus tests on basic mechanical performance, dielectric (e.g. insulating) properties, ageing treatments and thermal tests. The basic mechanical performance tests cover tensile strength and elongation at break, and puncture resistance. For the tensile strength tests a standard tensile test machine is used to gradually extend dumbbell shaped specimens that have been cut from the gloves. During this process both the force applied to the specimen and the extension of the specimen are recorded. The test is carried out using samples that have been subject to a pretreatment, the results for which are compared to those which have not. Several performance levels are available for the dielectric or electrical insulation tests (see Table 1). The thermal requirements consist of a flame retardancy test on the glove fingers and a cold crack test carried out after exposure to a -25° C environment. Optional tests for gloves with special properties together with their associated marking codes are listed in Table 2. The ozone pre-treatment consists of storing samples, such as those cut from gloves, in a specialised chamber containing equipment for generating ozone at set concentrations, while also controlling temperature and humidity within the chamber. Glove samples are stored in the chamber for a defined period of time before being visually assessed and subjected to electrical proof tests to assess any possible degradation. An ozone pre-treatment is included in the standard as ozone can be generated by high voltage sources and therefore gloves being worn in such an environment may be exposed to ozone.
Most European tests for insulation resistance assess performance in what is considered to be a worst case situation, e.g. when the sample is wet, as moisture will usually aid the flow of current. This means the test protocol often includes a pre-treatment procedure to maximise the moisture content of the product. This is either achieved by storage in an environment with a defined high humidity, or, as is the case in EN 60903, by immersion in water. SATRA has recently extended its high voltage testing capability from an in house designed machine generating 30 kV up to a new enlarged test chamber that is capable of generating voltages of up to 50 kV. While the step from 30 kV to 50 kV may not sound too great, it has required a complete change in the technology to be applied. The new equipment incorporates a variable voltage transformer that can be continuously ramped up from zero to a maximum of 50kV. Output current options enable a trip current of up to 40 milliamp to be applied. The cabinet also houses a water tank so that tests can be carried out under wet or dry conditions. Electrical interlocks ensure the machine can only be operated when the enclosure door is shut and the voltage controller set to zero initially. A motorised rheostat is used to increase the applied voltage at a pre-set rate, while the voltage and leakage current are monitored via two digital meters. The system also incorporates an extraction system to remove any ozone generated by the high voltage electrodes, as ozone is not only harmful to health but can also distort test results by providing unwanted paths due to its conductive nature. Considerable flexibility has been built into the cabinet to enable a wide range of PPE to be tested, including gloves and footwear to European and global standards. The new test cabinet greatly increases SATRA’s ability to test PPE intended to protect against high voltages, and readers wishing to know more about its capability should contact Daniel Harrison or Peter Doughty at SATRA.
Published: 01st Jul 2012 in Health and Safety International