BioElectrionix Hub: Symbols commonly used in medical devices

Electrical equipment is classified into different classes based on the type of protection used to prevent electric shock. For equipment that runs on mains power, there are generally two levels of protection - basic and supplementary.

Basic protection refers to the primary insulation and grounding of the equipment, which is designed to prevent electric shock under normal operating conditions. This includes the use of double insulation, reinforced insulation, or earth grounding.

Supplementary protection, on the other hand, is an additional layer of protection that is intended to come into play in the event of failure of the basic protection. This may include the use of fuses, circuit breakers, residual current devices (RCDs), or other protective devices that can detect and interrupt electrical faults or overloads.

In summary, electrical equipment is categorized into different protection classes based on the safety features used to prevent electric shock, and supplementary protection is an additional safety measure designed to provide backup protection in case of a failure of the primary or basic protection.

Symbols used in medical device

Class I equipment

Class I equipment has a protective earth. The basic means of protection is the insulation between live parts and exposed conductive parts such as the metal enclosure. In the event of a fault that would otherwise cause an exposed conductive part to become live, the supplementary protection (i.e. the protective earth) comes into effect. A large fault current flows from the mains part to earth via the protective earth conductor, which causes a protective device (usually a fuse) in the mains circuit to disconnect the equipment from the supply.

It is important to realise that not all equipment having an earth connection is necessarily class I. The earth conductor may be for functional purposes only such as screening. In this case the size of the conductor may not be large enough to safely carry a fault current that would flow in the event of a mains short to earth for the length of time required for the fuse to disconnect the supply.

Class I medical electrical equipment should have fuses at the equipment end of the mains supply lead in both the live and neutral conductors, so that the supplementary protection is operative when the equipment is connected to an incorrectly wired socket outlet.

Further confusion can arise due to the use of plastic laminates for finishing equipment. A case that appears to be plastic does not necessarily indicate that the equipment is not class I.

There is no agreed symbol in use to indicate that equipment is class I and it is not mandatory to state on the equipment itself that it is class I. Where any doubt exists, reference should be made to equipment manuals.

The symbols below may be seen on medical electrical equipment adjacent to terminals.

Symbols seen on earthed equipment.

Class II equipment

The method of protection against electric shock in the case of class II equipment is either double insulation or reinforced insulation. In double insulated equipment the basic protection is afforded by the first layer of insulation. If the basic protection fails then supplementary protection is provided by a second layer of insulation preventing contact with live parts.

In practice, the basic insulation may be afforded by physical separation of live conductors from the equipment enclosure, so that the basic insulation material is air. The enclosure material then forms the supplementary insulation.

Reinforced insulation is defined in standards as being a single layer of insulation offering the same degree of protection against electric shock as double insulation.

Class II medical electrical equipment should be fused at the equipment end of the supply lead in either mains conductor or in both conductors if the equipment has a functional earth.

The symbol for class II equipment is two concentric squares illustrating double insulation as shown below.

Symbol for class II equipment.

Symbol for class II equipment

Class III equipment

Class III equipment is defined in some equipment standards as that in which protection against electric shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present. SELV is defined in turn in the relevant standard as a voltage not exceeding 25V ac or 60V dc.

In practice such equipment is either battery operated or supplied by a SELV transformer.

If battery operated equipment is capable of being operated when connected to the mains (for example, for battery charging) then it must be safety tested as either class I or class II equipment. Similarly, equipment powered from a SELV transformer should be tested in conjunction with the transformer as class I or class II equipment as appropriate.

It is interesting to note that the current IEC standards relating to safety of medical electrical equipment do not recognise Class III equipment since limitation of voltage is not deemed sufficient to ensure safety of the patient. All medical electrical equipment that is capable of mains connection must be classified as class I or class II. Medical electrical equipment having no mains connection is simply referred to as "internally powered".

Equipment types

The degree of protection for medical electrical equipment is determined by its type designation, which takes into account the specific safety requirements of the equipment based on its intended use. This is because different types of medical electrical equipment are used in different applications and may require different levels of electrical safety.

For example, certain medical electrical equipment may require direct cardiac connection, which would require a higher level of safety than equipment that does not require this type of connection. Type designations are used to ensure that each piece of medical electrical equipment is designed and manufactured to meet the appropriate safety requirements for its intended use.

By using type designations, medical electrical equipment manufacturers can ensure that their products meet the necessary safety standards for their specific application. This helps to prevent electrical hazards and ensure the safety of patients and healthcare professionals who use the equipment.

symbols and definitions for each type shown below for classification of medical electrical equipment.

Type	Symbol	Definition
B		Equipment providing a particular degree of protection against electric shock, particularly regarding allowable leakage currents and reliability of the protective earth connection (if present).
BF		As type B but with isolated or floating (F - type) applied part or parts.
CF		Equipment providing a higher degree of protection against electric shock than type BF, particularly with regard to allowable leakage currents, and having floating applied parts.

All medical electrical equipment should be marked by the manufacturer with one of the type symbols above.

Lets see a bit more about Defibrillator !

Defibrillation is a medical procedure that involves delivering an electric shock to the heart to restore its normal rhythm in case of a life-threatening cardiac arrhythmia, such as ventricular fibrillation or pulseless ventricular tachycardia. The electric shock interrupts the chaotic electrical activity of the heart and allows the heart's natural pacemaker to resume control and restore a regular heartbeat.

Defibrillation is typically performed using a medical device called a defibrillator, which delivers a controlled electric shock to the heart through paddles or patches attached to the chest. There are two main types of defibrillators: external and implantable.

External defibrillators are used in emergency situations, such as cardiac arrest, and are usually found in hospitals, ambulances, and other healthcare facilities. They can be further classified into manual and automated external defibrillators (AEDs), with the latter being designed for use by non-medical personnel, such as first responders or bystanders.

Implantable defibrillators, also known as cardioverter-defibrillators (ICDs), are small devices that are surgically implanted under the skin of the chest or abdomen. They are used to monitor the heart's rhythm and deliver electrical shocks if needed to restore normal heart function. ICDs are typically recommended for people who are at risk of sudden cardiac death due to certain medical conditions, such as heart failure, arrhythmias, or previous heart attacks.

Both external and implantable defibrillators are designed and tested to withstand various environmental conditions and meet specific safety standards to ensure their efficacy in delivering life-saving electric shocks.

patient monitors ECG machines are designed with defibrillation proof function means you may deliver Electric shock to the patient without removing the attached electrodes to the patient