Feature Article

DESIGN

Resolving risk

After more than 13 years, and four years after it was formerly issued, the third edition of IEC 60601, Medical Electrical Equipment, Part 1: General Requirements for Basic Safety and Essential Performance, is at its final stage before it becomes operational.

It will form part of the certification scheme of the International Electrical Commission System for Conformity Testing and Certification of Electrotechnical Equipment and Components (IECEE). However, many issues are still not totally resolved by the IECEE and it is working intensively to settle these. These include risk management according to ISO 14971 Medical Devices, Application of Risk Management to Medical Devices, and how it relates to IEC 60601-1 3rd edition. Manufacturers, regulators, test houses and other stakeholders have invested considerable time and resources in training and implementation of the new standard. In addition, it is generally accepted that success of the 3rd edition requires the standard to become operational in the near future.

The start of IEC 60601-1

IEC 60601-1 has existed for more than 30 years and is a well-proven standard that ensures safety in the health sector worldwide. The first
edition, published in 1977, was based
on the framework outlined in IEC 513,
Basic Aspects of the Safety Philosophy
of Electrical Equipment Used in Medical Practice. IEC 513 was published in 1976 and provided fundamentals
for the future IEC 60601-1. One thing that differentiates medical equipment from all other electrical
equipment is the direct electrical contact with patients that is required for the equipment to perform monitoring, treatment and diagnostics. The differing states of patients make that electrical contact even more complicated and the standard must take that into consideration. The patient may be in a weak state, in constant electrical
connection with life sustaining equip- ment and may even be unconscious.

Ensuring patient insulation

It is clear that IEC 60601-1 must mitigate the special conditions of the patient. One of the important tools in the standard is Patient Leakage Current, which is the stringent limit placed on the unintentional current flowing from the medical equipment to the patient. Patient circuits in direct electrical contact with the patient via electrodes or catheters are controlled from special electrical circuits that have an additional separation from all other electrical circuits. These circuits are electrically isolated from Earth potential and are referred to as Type BF or Type CF circuits, having one or more Type BF or Type CF Applied Part(s). A Type BF circuit shall not exceed 100 µA (a.c) of Patient Leakage
Current during normal operation, and Type CF is limited to 10 µA (a.c) of Patient Leakage Current during normal operation. Other standards including IEC 60950, Information Technology Equipment Safety, and IEC 61010 Electrical Equipment For Measurement, Control and Laboratory Use, allow leakage current up to
3500 µA (a.c), which could cause fatal situations in medical equipment applications.

Components such as transformers, optocouplers, relays and operational amplifiers with insulation are used to provide the necessary insulation to avoid excessive leakage current and ensure adequate creepage and clearance distances.

The first edition of IEC 60601-1:
1977 differentiates creepage and clearance distances between patient circuits and operator circuits. The standard’s Tables XVI and XVII together with clauses 20.1 and 20.2 described how to calculate creepage and clearance values for all types of circuits. The second edition of the standard (1988) combined and simplified the approach based on the highest values of Table XVI in the first edition, and the second edition had one table for creepage and clearance.

History often repeats itself and the third edition of IEC 60601-1 published in 2005 reintroduced the original idea used in the first edition,
except, this time the rationale was based on insulation coordination used in IEC 60950. The philosophy of insulation coordination is clear and well defined, but it is complex in its application. The system is based on two new definitions: Means of Operator Protection (MOOP) and Means of Patient Protection (MOPP).

Creepage and clearance values for
patient circuits (MOPP) are now found
in Table 12 in the 3rd edition and are the same as for the first and second
edition, except that the reference
voltage now goes up to 10000 V(a.c).
MOOP is for circuits that may be
accessible to the operator (for example,
doctor or nurse), but not to the patient. A simplified method is to ensure a safe distance between the medical equipment and the patient. This is defined as Patient Environment and shown in the Figure A.9 of IEC 60601-1 3rd edition. MOOP follows most of the general rules of IEC 60950 and is intended to provide more freedom and reduce costs for the benefit of society.

So where is the problem?

To determine the acceptability of any component transformer used in a medical device, IEC 60601-1 3rd
edition refers to IEC 61558-1 Safety of Power Transformers, Power Supplies,
Reactors and Similar Products. It is useful to follow the logic chain of
the clauses between IEC 60601-1
and IEC 61558-1, as follows.

Transformer construction requirements are specified in clause 15.5.3 of IEC 60601-1. The clause redirects to IEC 61558-1, clause 5.12, which again outlines multiple clauses applicable to associated transformers, one of
which is clause 26 Creepage Distance and Clearances. Clause 26 redirects to Table 13, Tables C and D. However, the values are lower in Table 13 and Tables C and D in IEC 61558-1 than the
values in Table 12 in IEC 60601-1. This, therefore, compromises clause 8.5 of IEC 60601-1. In other words,
transformers that conform to IEC 61558-1 comply with MOOP, but not to MOPP as outlined above.

As a result of the reference to IEC 61558-1 in IEC 60601-1:2005, it is possible to make the interpretation that an IEC 61558-1 approved transformer automatically fulfills the fundamental requirements of IEC 60601-1. It is a clear intent of IEC 60601-1 3rd edition to maintain the overall Patient Insulation
philosophy as defined above. The 3rd edition of the standard clearly distinguishes between MOPP and MOOP because MOPP has proven to be a safe approach since the standard originated more than 30 years ago. The reference in clause 15.5.3 to IEC 61558-1 was intended to ease the job for manufacturers in selecting “off the shelf transformers” for their designs. The rationale to clause 15.5.3 supports this by stating:

“The requirements specified in IEC 61558-1, Subclause 5.12 are generally similar to those in the second edition of this standard, but transformers complying with them are likely to be more readily available.”

IEC 61558-1 covers MOOP properly, but not MOPP.

Transformer testing

Transformers used in medical equipment applications are developed in many shapes and sizes for various purposes. Examples include:

• transformers used to insulate floating circuits (Type BF and CF circuits), which prevent hazardous leakage
  current from entering the patient
• high voltage step-up transformers used for X-ray applications, magnet stimulators and laser applications
• conventional mains transformers, which are incorporated in 90% of all medical equipment.

It is common for these types of transformer to have high insulation properties and/or extreme thermal conditions.

The medical equipment industry has always been at the leading edge of design and transformer technology is no exception. Validation and testing of new transformer technology requires “out of the box” thinking and the answer to this is risk management. The 3rd edition of IEC 60601 has implemented the managed risk philosophy nearly everywhere in the standard, including for transformers used in medical devices.

Look for requirements in IEC 61558 or in IEC 60601?

As shown above, it is necessary to look in two standards to find construction requirements for transformers.

The general concept in IEC 60601-1 is that all testing requirements are covered by IEC 60601 and all construction requirements should be covered by IEC 61558. The following example shows an area where the two standards interact.

The heating test. This is a temperature test that is applied to the entire device and it verifies that temperature values of critical components are below the predefined limits set by the standard. Clause 11.1.3 (d) of IEC 60601 has instructions for this test, but following the reference of clause 15.5.3 in IEC 60601 to clauses 5.12 to 5.14 in IEC 61558, a new set of requirements for the heating test for transformers with different temperature limits would seem to apply. Clause 5.12 does, however, indicate that the test may be conducted according to the relevant product standard, which in this case is IEC 60601 3rd edition, following the conditions outlined therein.

This example is not unique. In fact, all testing requirements and several construction requirements for transformers specified in CL.5.12 of IEC 61558 require both IEC 60601-1 and IEC 61558-1 to determine which requirements to apply.

Moving forward

In view of this, it is important that all stakeholders of the standard are consistent and take appropriate measures. Engineers involved in development of medical electrical equipment in accordance with IEC 60601-1 3rd edition are in immediate need of short-term and long-term solutions to avoid potentially hazardous designs as a result of misinterpretation of the requirements in the standards. There are several ways to ensure consistency and safety of devices in the future.

Amendment 1. IECEE is already working on the first Amendment to IEC 60601-1 3rd edition, which is scheduled to be issued in 2012. The Amendment addresses any issues identified with the standard, but because there may be several issues raised by stakeholders, it may be some period of time (up to 2012) before Amendment 1 is implemented. Therefore, this is a long-term solution. Amendment 1 may address the transformer question in one of the following ways:

• expansion of clause 15.5.3 to include all construction requirements and revise existing test clauses to explicitly cover transformers and remove the reference to IEC 61558-1 by deleting clause 15.5.3, or
• introduce a new annex to the standard that outlines all construction and testing requirements for medical transformers so that they would comply with clause 8.5, or
• take an alternative approach.

CTL decisions. The IECEE incorporated a mechanism for clarifying open issues raised for standards not operating as intended. The mechanism is the Committee of Testing Laboratories (CTL), which “deals with questions of practice related to the test specifications and test methods detailed in the standards accepted for use in the IECEE, and to constructional details of test equipment referred to in these standards,” as defined by the IECEE.

Decisions taken by the CTL are intended for clarification of the standard only, which means that the CTL cannot change any requirement in the standard; it can only provide guidelines on how to apply and interpret clauses in the standard. The CTL decision may be included in Amendment 1 so that the standard makes a clear reference to applicable clauses of IEC 61558-1 and IEC 60601-1. In this way, users of the standard are guided by clear instructions. The CTL decision on this and other issues should also give guidelines on where it may be appropriate to apply risk management. In addition, the CTL decision may specify a new Technical Report Form designed specifically for medical transformers.

IEC 61558-2-xx for Medical Transformers. The introduction of IEC 61558-1 states the need for usage of a particular requirement standard (identified as the Part 2) in conjunction with the general requirement standard (identified as the Part 1).

“The object of Part 1 of IEC 61558 is to provide a set of requirements and tests which are considered to be generally applicable to most types of transformers, and which can be called up as required by the relevant Part 2 of IEC 61558. Part 1 is thus not to be regarded as a specification by itself for any type of transformer, and its provisions apply only to particular types of transformers to the extent determined by the appropriate Part 2.”

Currently there are more than 15 requirement standards that have been developed for use with the IEC 61558-1. Therefore, to be consistent with the above referenced guidance information and the existence of other Part 2s for IEC 61558, it would appear reasonable to consider the development of an IEC 61558 Part 2 specifically for medical transformers.

In summary, the current reference to IEC 61558 in IEC 60601-1 has introduced an opportunity for misinterpretation that may result in MOPP not being adequately addressed and a potential source of harm to patients. It is recommended that actions be undertaken to quickly address this potential source of harm by manufacturers, standards developers and test houses.

Is IEC 61558-1 the right standard?

The question, of course, is whether IEC 61558-1 is appropriate for the design of both traditional transformers and new state of the art transformers designed by cutting edge technology. One of the objectives behind the new 3rd edition of IEC 60601-1 was to overcome the problem of addressing new technology that is not covered by previous editions of the standard. The risk management file is the new tool supporting new technologies, but for some reason this tool has not been applied rigidly for the transformer component.

The transformer problem does not have an easy solution as the following statement in the first edition of IEC 60601:1977 noted more than 30 years ago, “Requirements for transformers to be used in medical electrical equipment will be dealt with in a future separate IEC standard.” It is hoped that it will not take another thirty years to solve the issue!

Ken Dybdahl is Senior Project Engineer, Medical Electrical and Laboratory Equipment, UL International Demko A/S, Lyskaer 8, DK-2730 Herlev, Denmark, tel. +45 44 85 62 97, e-mail: ken.dybdahl@dk.ul.com, www.ul-europe.com