Electrical Safety Testing (EST) to IEC 62353
IEC 62353:2014 applies to the testing of medical electrical equipment and medical electrical systems, hereafter referred to as ME equipment and ME systems, or parts of such equipment or systems, which comply with IEC 60601-1:1988 (second edition) and its amendments and IEC 60601-1:2005 (third edition) and its amendments, before putting into service, during maintenance, inspection, servicing and after repair or on occasion of recurrent tests to assess the safety of such ME equipment or ME systems or parts thereof.
For equipment not built to IEC 60601-1 these requirements may be used taking into account the safety standards for the design and information.
This EST guide covers a basic introduction to electrical safety, definitions of a medical electronic device, the IEC 60601 standard and an in-depth overview of the IEC 62353 publication. The structure and topics discussed in this guide are written in a way such that the widest possible audience can benefit.
Electrical current is a secondary energy form consisting of the flow of charge (in coulombs) through a circuit over a certain time period, and is depicted in ampere.
1 = Q/t or 1 Ampere = 1 Coulomb/1 second
When electrical current passes through a conductor or electrical circuit, it generates an electrical potential (depicted in volt), see figure 1.
Figure 1: Ohm's Law
There is a directly proportional relationship between the electrical current (ampere) through and the electrical potential (volt) across the conductor (ohm). This is commonly known as Ohm’s law.
V (a – b) = I * R
The force required to deliver the electrical current across a potential difference is known as power, which is represented in Watt’s. Power is a product of voltage (volt) and current (ampere):
Another factor of electricity is electrical energy (joules), a product of electrical power (watts or joules/second) and time (seconds). The relationship is provided below:
The relationship between current, voltage and resistance can be equally applied to water running through a pipe. In both cases, electric current and water prefer the path of least resistance. The larger the cross section of the pipe or conductor, the easier water or current can flow at a certain water pressure or voltage. (See figure 2.)
Figure 2 Example showing water following path of least resistance
The thinner or longer the water pipe, the more water pressure is required to deliver the same gallons per minute (flow or current).
The thinner or longer the conductor (assuming a particular specific resistance of material), the more voltage is required to deliver the same current.
1.2 The Human Body
A significant part of the human body is made up of water along with dissolved ions and minerals, which are capable of conducting electrical currents. Broadly speaking, the hazard of such electrical currents would depend on: Strength of the current, Path of the current, Total impedance for the current path, Frequency of the current, Duration of the current being applied, Electrical currents can be extremely dangerous to the human body. The energy (power and time factor) released by electrical current passing through human tissue can generate burns and excite or stimulate muscles of the respiratory system (intercostals).
The most critical muscles are those in the human heart, which are driven (excited) by very tiny amounts of electrical currents. When the heart is exposed to external electrical currents (electrical shock), the heart can lose its normal sinus rhythm, required to sustain a healthy blood circulation, and move into ventricular fibrillation. This stops the circulation of oxyhaemoglobin (oxygenated blood cells) to the brain and organs and when left untreated, will result in death within 15 minutes.
Ironically, the most common treatment of ventricular fibrillation is the use of a defibrillator which delivers a very high current pulse, up to 100A across the heart. The energy in that high current pulse is sufficient to temporarily clamp the heart muscles (ie. stop the heart completely) before releasing it again and allowing the heart to resume in its normal sinus rhythm.
Consider the following examples of a macro shock showing the effect of a 50 / 60 Hz current on the human body when applied to the skin for 1 – 3 seconds (non-invasive):
|0.5 -1.1 mA||Current just noticeable when applied to the finger tip|
|6 – 16 mA||Painful shock, unable to let go, cannot be tolerated over 15 minutes|
|75-400 mA||Ventricular fibrillation, respiratory arrest, leading to death|
|<1 A||Serious burns and muscular contraction of such a degree that the thoracic muscles constrict the heart|
(Data adapted from published research by Professor C.F. Dalziel)
The graph below (figure 3) highlights the different effects of electrical current on the human body as understood by Dr. Howard M. Hochberg.
Figure 3: Impact of current on the human body, adapted from research by Howard M. Hochberg, 1971
1.3 IEC 60601 Body Model
To ensure a standardized method of simulating the impedance of the human body, measurement circuits have been designed to simulate the average, typical electrical characteristics of the human body. These measurement circuits are referred to as a Body Model or Measuring Device (MD in IEC 60601-1). The main impedance is formed by a 1kΩ resistor, shown in figure 4.
Figure 4: Example of a measuring device MD to IEC 60601
Part two to follow...