SCOTTISH HEALTH GUIDANCE NOTE
Servicing and repair of medical electrical equipment:
safety during hazardous live testing
September 2004
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Servicing and repair of medical electrical equipment: Safety during hazardous live testing
What is this guidance about? This Scottish Health Guidance Note contains advice on the means of achieving safety during hazardous live testing of medical electrical equipment. It provides guidance on the prevention of danger to test personnel and other persons. The guidance is based on HSE guidance leaflet INDG354 (1).
What are the hazards? In addition to injuries from falling objects, moving parts, pressurized systems, high and low temperatures, chemicals, ionizing and non-ionizing radiation etc., a significant danger to people carrying out live testing of medical electrical equipment is electric shock or burns. Electric shock can lead to serious or fatal injury, either due to the current itself or from injuries sustained when a person reacts to an electric shock, for example by falling or touching another hazard. Burn injuries may result from current flowing through the body, including high frequency ‘diathermy’ currents, or as a result of arcing when conductors are accidentally short-circuited. Medical electrical equipment such as defibrillators may also contain dangerous levels of stored electrical energy.
What is hazardous live testing? Any situation where contact with exposed live parts is possible should be considered hazardous. However, live parts are not considered to be hazardous if one of the following conditions, defined in BS EN 50191 (2), is met:
(a) The voltage at frequencies above 500Hz does not exceed 25V a.c. or 60V d.c. and complies with the requirements for SELV or for PELV as defined in BS7671 (3).
(b) In the case of voltages at frequencies up to 500Hz exceeding 25V a.c. or 60V d.c., the resultant current through a non-inductive resistance of 2k. does not exceed 3mA a.c. (r.m.s.) or 12mA d.c.
(c) At frequencies above 500Hz no hazardous body currents or contact voltages occur. (Permissible body current and contact voltage values at different frequencies may be found in Table A1 of the standard).
(d) The discharge energy does not exceed 350mJ.
How can injuries happen during testing? Any activity where there is a risk of direct or indirect contact with exposed conductors that are live at hazardous voltages may result in an electric shock. Simultaneous contact of a part of the body with a conductor that is hazardous live (e.g. connected to the mains supply) while another part of the body is in contact with a return path (e.g. earthed metalwork) may result in electric current passing through the body. If the current is high enough, this will be experienced as electric shock.
The risk of electric shock is highest where:
Equipment to be repaired contains a hazardous fault e.g. short circuit, fluid ingress, earth-fault, loose component or damaged insulation.
Measurements or adjustments are made with normal protective covers removed.
The equipment under test produces a hazardous output, e.g. surgical diathermy.
There is a deliberately applied fault condition or test voltage during electrical safety tests.
The equipment (e.g. a defibrillator) contains stored energy.
1Servicing and repair of medical electrical equipment: Safety during hazardous live testing
The risks may be further increased by: Equipment with large areas of earthed metal – easy to touch at the same time as touching a live part.
Sharp conductors or components - shock current will increase if the sharp end of a conductor penetrates the skin.
Handling circuit boards which generate hazardous voltages or have stored energy, e.g. flash units.
Inappropriate test leads - fail to provide adequate isolation from live parts.
Inappropriate or incorrectly used test equipment - may expose the tester to hazardous voltages (see oscilloscopes).
Carrying out work on a ward or in a patient’s home - other staff, patients or visitors (including children) are put at risk and their actions may place the tester at risk.
An electrical installation, such as in a patient’s home, that is not electrically sound with regard to earthing, insulation and polarity.
Controlling the risk: The risk of electrical injury during hazardous live testing can be reduced to an acceptable level by:
working in a designated test area, either permanent or temporary, that satisfies the principles set out in this Guidance Note; and
the work being carried out by competent staff using safe systems of work, including the use of appropriate tools, test equipment and personal protective equipment, on the basis of a suitable risk assessment.
The following paragraphs expand on these general principles:
Carrying out a personal risk assessment Before starting hazardous live testing of medical electrical equipment, the tester needs to assess the risk of injury, damage to property or equipment, the effect on other people and the effectiveness of precautions or protective measures. The tester should bear in mind factors which might increase the risk, especially those given in this Guidance Note. When carrying out a risk assessment for electrical testing, the tester should ask himself or herself the following questions:
Are there other hazards that need to be considered, e.g. contact with moving parts, laser emissions, high temperatures?
Can the work be done with the equipment isolated from the electrical supply? (Repairs and replacements of faulty parts can and should be done with equipment isolated from the supply.)
Can I work unimpeded and without distraction?
Is there a safer method of faultfinding or adjustment, e.g. software diagnostics?
What voltage or current is likely to be present?
Has stored charge had time to dissipate?
Are there any special electrical hazards, e.g. high voltage (>1kV), high frequency etc?
Is the right test equipment available, with leads?
Have persons not involved in testing been protected from danger?
Is it possible to carry out the work in a safer place?
Should an isolating transformer and / or Residual Current Device (RCD) be used?
Can additional measures be employed to prevent contact with live parts?
Are there components, such as capacitors, which may retain significant charge that should be allowed to dissipate or require special arrangements for discharging?
Do I need to use eye protection or other personal protective equipment (PPE)?
Am I competent for that type of work, or am I adequately supervised?
If the risk assessment concludes that hazardous live testing must be carried out, it will be necessary to decide on the precautions to be taken against injury.
What precautions should be taken? Where possible, the work should be done with the equipment isolated from the supply (this is a requirement of the Electricity at Work Regulations 1989). If this is impossible or unreasonable, one or more of the following may be employed to prevent contact with exposed live parts:
Replace the equipment covers for the duration of the test.
Attach and detach measuring probes only with the equipment under test (EUT) isolated from the supply.
Use temporary insulation, or modified covers, to allow access for adjustment while maintaining protection against contact with hazardous live parts.
Use instruments and test leads which provide adequate protection / isolation for the voltages measured.
Prevent unauthorized people from approaching equipment under test by the use of signs and barriers or other appropriate access control. (This must not obstruct any escape route.)
If hazardous live parts cannot be shrouded safely and reliably, work should be carried out in a test area, constructed in accordance with EN 50191 (2), and as described below.
Test Areas A test area may be permanent, such as a room or part of a workshop, or temporary such as created around equipment at or near its point of use.
General Warning signs should be placed at the entrance to the workshop and at the live test area.
The workshop should have on display an Electric Shock First Aid Guide.
All test areas must restrict access by unauthorised persons.
Physical barriers may be necessary if site location, signs and monitoring by staff do not provide sufficient control over entry.
Only those persons who have received adequate instruction regarding the hazards should be permitted to enter a test area.
Any person working in a test area must be fully aware of the existing hazards and has a duty to take safety precautions in order to protect both themselves and others from risk. This should include using appropriate PPE.
In general, a temporary test area may require greater effort to create and make safe compared with using a permanent test area and should only be used where equipment is fixed or cannot reasonably be moved to a permanent test area.
Permanent test areas To reduce the risk of electric shock, hazardous live testing areas should be as earth free as possible.
Construction There should be no earthed metalwork accessible to someone in the work area.
Insulating material such as wood should be used to construct the workbench and any exposed metalwork such as trunking or service pipes should be covered with an insulating shield,
3Servicing and repair of medical electrical equipment: Safety during hazardous live testing
even if not earthed. Unearthed metalwork still presents a risk if it could be earthed inadvertently (e.g. by contacting an earthed object).
Cover plates for electrical supplies must be non-conducting, e.g. plastic.
The floor must be covered with an insulating material such as vinyl or a rubber mat to BS921 (4).
Plaster walls are acceptable. Metal shelves could provide an earth path and should be avoided.
Electrical supply An electrical supply derived from an isolating transformer complying with BS EN 61558-2-4(5) should be provided for the equipment under test. The two-wire output is taken to a mains socket outlet clearly marked ‘Isolated, earth-free, for test purposes only’. The earth terminal of this outlet must be isolated.
If socket outlets having earth-referenced supplies are required in the test area, these must be protected by an RCD with a tripping current of no more than 30mA. The RCD should be tested before each period of use. The supply outlets must be clearly labelled ‘earthed supply’, be non-conducting and have a protective earth connection. These outlets should be used with caution (see - Is the test area safe? and also Surgical diathermy).
An emergency stop button must be provided adjacent to the work area, so that all electrical supplies to the area (except lighting) can be interrupted by others in the event of an emergency.
Additional isolated supplies from a separate isolation transformer may be required to power test equipment. Portable isolating transformers may be suitable for this purpose, but there must be no earth on the outlet from the transformer.
Note: oscilloscopes should
Access and signageAccess to the live test area must be restricted. Precautions may include doors, barriers or gates, combined with suitable signs. If the workshop itself, and position of the live test area within it, provides good level of restriction of entry, the live area may require no further physical barrier.
Signs should be displayed at the test area, declaring the purpose of the facility and warning of the danger. It may be considered appropriate to provide an illuminated warning which is switched on when the area is in use. However, the presence of a person working in the area may provide sufficient indication that live testing is in progress.
Typical signs would include ‘no unauthorised entry’ and ‘danger of electric shock’. ‘High voltage test area’ should be displayed at the test area itself.
Temporary test areasTemporary test areas should be created where hazardous live testing is to be carried out on fixed equipment or equipment which cannot reasonably be moved to a permanent test area.
Risk AssessmentA risk assessment must be carried out prior to any live testing being undertaken. A personal risk assessment must be carried out in all temporary test areas prior to work commencing. Written generic risk assessment should be completed by the healthcare organisation for areas where hazardous live testing is regularly carried out.
Signage / Barriers In any location where hazardous live testing is taking place, signage and barriers must be put in place to ensure that members of the public and staff not involved in testing are prevented from entering the area. In certain areas where work takes place in a specific room then the doors to that room (with a sign attached to them) could form the barrier.
Lone Working Working alone on exposed or hazardous live equipment, including work by contractors, should not be done. At least one other (safety competent) person must remain in visual and audible contact throughout the period of live testing. This person must be aware of the risks and be able to recognize any dangerous conditions, actuate the emergency switch-off device and take any other appropriate action.
Test equipment for hazardous live testing Equipment used for live testing must be safe for that purpose. It is preferable to use commercially manufactured equipment, designed and specified for the voltages that may be encountered during use. Purpose-built in-house test equipment must be thoroughly risk assessed by a competent team during the design and manufacture processes. Test equipment should comply with BS EN 61010-1 (6).
Probes Probes used to access live parts are specified in GS38 (7). They have a minimal area of exposed metal at the tip. The maximum allowable dimension is 4mm with 2mm preferred. Retractable shrouds may improve safety, and finger guards must be incorporated. A fuse should be incorporated in the probe, unless adequate over-current protection is provided by the test equipment.
Oscilloscopes Conventional oscilloscopes have their signal common referred to protective earth. Therefore connecting a conventional oscilloscope to isolated equipment under test (EUT) would once more reference the EUT to earth and increase the risk of electric shock. Moreover, this means that all measurements have to be made with respect to earth and this can create measurement problems. However, defeating the protective earth in order to make ‘floating’ measurements either by disconnecting the protective earth or using an isolating transformer will allow the chassis to assume the potential of the probe earth lead. This provides a risk of electric shock and may stress the oscilloscope insulation. Manufacturers advise that defeating the protective earth is an UNSAFE AND DANGEROUS PRACTICE AND SHOULD NEVER BE DONE.
A battery-operated isolated-input oscilloscope or an oscilloscope with high voltage differential probes would provide safer working and is recommended.
General Battery operated test equipment with insulating case and controls are recommended for hazardous live testing. Such equipment will naturally be isolated until connected to the EUT. If the EUT is powered from an isolated supply, the whole system and working environment will be earth free. This should provide considerable risk reduction.
Special considerations Cathode ray tubes (CRTs) Special consideration is required when testing high voltage equipment such as video monitors fitted with cathode ray tubes. The line output transformer on such equipment may produce voltages up to 30kV. Electronics in the power supply may be directly connected to mains.
Surgical diathermy The patient applied current is at a sufficiently high frequency that electrical shock would not occur, but it does present a significant risk of burns. Particular attention should therefore be paid to using appropriate test equipment, leads and connectors.
The risk of conventional phase to earth shock would usually be confined to the mains input part of the device and may be reduced by using insulation or isolating transformers. Internally generated voltages, such as supplies to power amplifiers, are likely to be present within a surgical diathermy unit and may be high voltage and referenced to the equipment chassis. An isolating transformer would provide little protection in this case but insulating material may be effective.
It should also be noted that some surgical diathermy units incorporate circuitry to monitor the isolation of the patient from earth. Powering the equipment via an external isolating transformer during maintenance may interfere with the operation / calibration of this safety circuitry with subsequent risk to the patient.
Stored energy (defibrillators) All defibrillators, including battery-operated devices, generate and store potentially lethal charge. It is therefore essential that the manufacturer’s safety advice is followed, power is disconnected and stored energy safely discharged before accessing any high voltage component.
Although a bleed resistor may be incorporated into the high voltage circuit this should not be relied upon as the sole method of ensuring that stored energy has been discharged.
Direct shorting across capacitor terminals (e.g. using a screwdriver) should be avoided whether or not a safe level of charge is measured or assumed. Shorting capacitor terminals should only be performed with a specially constructed probe with shrouded bleed resistors.
Similar advice would apply to other devices where stored energy exceeds 350mJ.
Earth faults A fault in any protective or functional earth on the EUT may render the device hazardous to the tester. Even when the supply is from an isolating transformer, there may be a risk of shock in the event of simultaneous contact between the enclosure of the equipment and a pole of the isolated test supply.
It is recommended that earth continuity and insulation resistance between phase / neutral and earth is checked before making the equipment live.
The equipment should be safety tested before being returned to service.
6Servicing and repair of medical electrical equipment: Safety during hazardous live testing
Soak testing All risks, including fire and electrical hazards, must be carefully considered before leaving equipment unattended. Equipment must not be left with hazardous live parts exposed or readily accessible.
Remote connections / control When equipment is controlled from a remote location, for example a control panel in another room, then a permit system such as a ‘Sanction-for-Test’ may provide a significant reduction in risk. Locking of controls and formal equipment handover may also be appropriate.
Is the test area safe? It has to be clearly understood by the tester that an isolating transformer does not prevent an electric shock should contact be made with both phase conductors simultaneously.
Using earth-free isolated supplies for both the EUT and the test equipment may present less risk of shock than working with all systems earthed. However, hazardous voltages may be produced between different parts of the system, which are not obvious, and which are consequently more dangerous than in an earthed system. For example, there is a risk of shock between the equipment enclosure and the isolated supply if the EUT has an earth fault.
RCDs provide supplementary protection only. They do not prevent electric shock, but limit the duration of phase to earth shocks to non-lethal levels. As with isolation transformers they do not prevent a phase to neutral shock should the tester come into contact with both conductors.
Checking the live testing area Flooring Insulated matting should be inspected for damage and tested by an appropriate (flash) test annually as part of a planned system of maintenance.
Isolating transformers Unless continuously monitored by a Line Isolation Monitor, the transformer must be checked on a regular basis to ensure that it provides isolation from earth. Insulation between the secondary windings and earth should be measured.
RCDs Portable RCDs should be checked before use by pressing the test button. Fixed RCDs should be checked weekly and all RCDs tested annually using an RCD tester to measure speed of operation and tripping current.
External contractors Competency Healthcare organisations must be sure that any engineers who attend on site where hazardous live testing is required are competent to work on the type of equipment and competent in general electrical safety. This may be confirmed in advance at the time of tendering for equipment or when agreeing service contracts.
Responsibilities The healthcare organisation is responsible for electrical safety within its premises. Senior staff within each department are responsible for ensuring that anyone working within their department who is likely to conduct hazardous live testing is aware of these guidelines and has been cleared as having the necessary level of competence / supervision.
Senior staff will also be responsible for ensuring that temporary test areas in their department are adequately controlled. This may include barriers and warning signs as well as the co-operation of departmental staff. Service contractors and healthcare employees involved in hazardous live testing will therefore be required to abide by these guidelines, including the prohibition on lone working.
Training and competency of test technicians All personnel who are required to carry out hazardous live testing should be instructed in safety requirements, safety rules and local instructions applicable to their work, and records of that training should be maintained.
Competence to undertake hazardous live testing should be formally assessed by the departmental manager or appropriate supervisor and only those deemed to be adequately skilled or instructed should be permitted to work in hazardous live testing areas. It should be noted that qualifications or experience alone do not constitute competency.
In addition to having the basic electrical / electronic qualifications for carrying out this type of work, staff must be able to demonstrate that they understand and are able to implement unsupervised the precautions described in this Guidance Note. In particular, they must:
understand the equipment on which they are required to carry out the hazardous live testing and the risks that will arise during the testing activity;
understand how to undertake a risk assessment for the work, identifying the hazards, risks and risk control measures;
understand the precautionary techniques that can be taken to prevent injury, including when to use isolated or earth-referenced supplies in the test area;
understand and be able to implement the safe working procedures necessary to reduce the risks to an acceptable level, including the use of appropriate tools, test equipment and personal protection equipment;
know the bounds of their competence and when they should not undertake hazardous live testing.
References 1) Safety in electrical testing at work - General guidance Leaflet INDG354 HSE Books 2002 ISBN 0 7176 2296 7
2) BS EN 50191:2001 Erection and operation of electrical test equipment
3) BS 7671:2001 Requirements for electrical installations. IEE Wiring Regulations, Sixteenth edition (incorporating Amds 1:2001, 2:2004).
4) BS 921:1976 Specification. Rubber mats for electrical purposes.
5) BS EN 61558-2-4:1998 Safety of power transformers, power supply units and similar. Particular requirements for isolating transformers for general use.
6) BS EN 61010-1: 2001 Safety requirements for electrical equipment for measurement, control and laboratory use.
7) Electrical test equipment for use by electricians GS38 HSE Books 2003 ISBN 0 7176 0845
