Electrical safety

Published on 07/02/2015 by admin

Last modified 07/02/2015

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Electrical safety

The electrical equipment used in the operating theatre and intensive care unit is designed to improve patient care and safety. At the same time, however, there is the potential of exposing both the patient and staff to an increased risk of electric shock. It is essential for the anaesthetist to have a thorough understanding of the basic principles of electricity, even though these devices include specific safety features.

In the UK, mains electricity is supplied as an alternating current with a frequency of 50 Hz. The current travels from the power station to the substation where it is converted to mains voltage by a transformer. From the substation, the current travels in two conductors, the live and neutral wires. The live wire is at a potential of 240 V (or more accurately 240 RMS (root mean square)). The neutral is connected to the earth at the substation so keeping its potential approximately the same as earth. The live wire carries the potential to the equipment whereas the neutral wire returns the current back to the source, so completing the circuit.

Principles of electricity (Fig. 14.1)

Identification of medical electrical equipment

A single-fault condition is a condition when a single means for protection against hazard in equipment is defective or a single external abnormal condition is present, e.g. short circuit between the live parts and the applied part.

The following classes of equipment describe the method used to protect against electrocution according to an International Standard (IEC 60601).

Class I Equipment

This type of equipment offers basic protection whereby the live, neutral and earth wires do not come into contact with each other. There is a secondary protection whereby parts that can be touched by the user, such as the metal case, are insulated from the live electricity and connected to an earth wire via the plug to the mains supply. There are fuses positioned on the live and neutral supply in the equipment. In addition, in the UK, a third fuse is positioned on the live wire in the mains plug. This fuse melts and disconnects the electrical circuit in the event of a fault, protecting the user from electrical shock. The fault can be due to deteriorating insulation, or a short circuit, making the metal case ‘live’. Current will pass to earth causing the fuse to blow (this current is called ‘leakage current’). Some tiny non-fault leakage currents are always present as insulation is never 100% perfect. A faultless earth connection is required for this protection to function.

Class II Equipment

This type of equipment, also called double-insulated equipment, has double or reinforced insulation protecting the accessible parts. There is no need to earth this type of equipment. The power cable has only ‘live’ and ‘neutral’ conductors with only one fuse.

Class III Equipment

This type of equipment does not need electrical supply exceeding 24 V AC or 50 V DC. The voltage used is called safety extra low voltage (SELV). Although this does not cause an electrical shock, there is still a risk of a microshock. This equipment may contain an internal power source or be connected to the mains supply by an adaptor or a SELV transformer. The power input port is designed to prevent accidental connection with another cable.

The following types of equipment define the degree of protection according to the maximum permissible leakage current.

Type B Equipment

This may be class I, II or III mains-powered equipment or equipment with an internal power source. This equipment is designed to have low leakage currents, even in fault conditions, such as 0.5 mA for class I and 0.1 mA for class II. It may be connected to the patient externally or internally but is not considered safe for direct connection to the heart.

The equipment may be provided with defibrillator protection.

Type BF Equipment

This is similar to type B equipment, but the part applied to the patient is isolated from all other parts of the equipment using an isolated (or floating) circuit. This isolation means that allowable leakage current under single fault conditions is not exceeded even when 1.1 times the rated mains voltage is applied between the attachment to the patient and the earth. The maximum AC leakage current is 0.1 mA under normal conditions and under a single fault condition is 0.5 mA. It is safer than type B but still not safe enough for direct connection to the heart.

The equipment may be provided with defibrillator protection.

Type CF Equipment

This can be class I or II equipment powered by mains with an internal electrical power source, but considered safe for direct connection to the heart. Isolated circuits are used. There is no risk of electrocution by leakage currents (allows 0.05 mA per electrode for class I and 0.01 mA per electrode for class II). This provides a high degree of protection against electrical shock. This is used in ECG leads, pressure transducers and thermodilution computers.

The equipment may be provided with defibrillator protection.

Attention!

The user must consult the accompanying documents for any equipment. A black triangle on a yellow background with an exclamation mark means that there is no standardized symbol for the hazard:

High voltage and risk of electrocution!

Protective Earth

The equipment itself has its own earth connection via the green-and-yellow lead in the conventional three-pin plug. The earth lead is connected to the external case of the equipment so reducing it to zero potential. Although this provides some protection, it does not guarantee it.

Functional Earth

This is part of the main circuit. The current is returned via the neutral wire back to the substation and so to earth. In effect, all conventional electrical circuits have functional earth. It is necessary for proper functioning of electrical equipment and is not a safety feature. On older equipment, the same symbol may have been used to denote protective earth.

Additional Protective Earth

This equipment carries an additional protective earth. This protects against electric shock in cases of a single fault condition.

Equipotentiality

This is used to ensure that all metalwork is normally at or near zero voltage. Therefore, under fault conditions, all the metalwork will increase to the same potential. Simultaneous contact between two such metal appliances would not cause a flow of current because they are both at the same potential, therefore no shock results. This provides some protection against electric shock by joining together all the metal appliances and connecting to earth.

Drip Proof, Splash Proof, Water Tight

Depending on the nature and use of the equipment, some are drip proof, splash proof or water proof.

Anaesthetic-proof equipment

AP equipment standards are based on the ignition energy required to ignite the most flammable mixture of ether and air. They can be used within 5–25 cm of gas escaping from a breathing system. The temperature should not exceed 200°C. It is a less stringent requirement.

Anaesthetic-proof equipment category G

APG standards are based on the ignition energy required to ignite the most flammable mixture of ether and oxygen. Can be used within 5 cm of gas escaping from a breathing system. The temperature should not exceed 90°C. This is a more stringent requirement as the energy level should be less than 1 mJ.

CE

This is one of the most important symbols. It means conformity according to the Council of Europe Directive 93/42/EEC concerning medical devices.

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Essentials of Anaesthetic Equipment 4e

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Principles of electricity (Fig. 14.1)

Electric current (I)

Potential difference or voltage (V)

Electrical resistance (R)

Impedance (Z)

Ohm’s law

Capacitance

Inductance

Identification of medical electrical equipment

Class II Equipment

Class III Equipment

Type B Equipment

Type BF Equipment

Type CF Equipment

Attention!

Protective Earth

Functional Earth

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Drip Proof, Splash Proof, Water Tight

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Anaesthetic-proof equipment category G

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Principles of electricity (Fig. 14.1)

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