During our laboratory inspections, we found that some laboratory users were using a type of variable transformer (Variac) which has an obvious potential hazard of electrocution (photos 1 &2). The obvious hazard with this kind of transformer is that it does not provide safe power connections, As seen in the photos, the power connection terminals (220 V input, Photo 1 0-250 V output) are all exposed, posing an obvious electrical hazard to the users and people nearby.

Photo 1

Photo 2

Photo 3

This type of transformer MUST NOT be used unless appropriate modification is made. The variac in photo 3 has been modified (by OLS) to provide safe power connections. However, modification (by a qualified person) should only be regarded as a short term measure. The long term measure is to purchase safe electrical equipment (in conformance with safety standards) in the first place. You should consult the Purchasing Office or SEPO if you need information regarding the supply sources for safe electrical equipment.

Electrical equipment is part of our daily lives. How well we are protected from being harmed by the electrical energy depends very much on the safety features built into the equipment.

The safety requirements for electrical equipment are covered in various standards such as the following British Standards:

BS 415 : Safety requirements for mains operated electronic and related apparatus for household and general use.

BS 3456 : Specification for safety of household and similar electrical appliances.

BS 4743 : Specification for safety equipment for electronic measuring apparatus.

Electrical equipment purchased through the Purchasing Office in HKUST are required to satisfy those standards. However, the safety standard of equipment purchased by individual users may not be as assured. In order to help users to identify the major safety features that are expected from a piece of "safe"electrical equipment, we have highlighted below some of those safety requirements which are regarded as essential and which can be readily identifiable by users.

 

Prevention of Electric Shock

• All live parts (those conductive parts which normally carry electric current) are enclosed to prevent accidental contact.
• The enclosure is of adequate strength and cannot not be broken easily.
• Access panels to live parts are securely fixed and cannot be opened without specific tools, such as a screwdriver. There should be warning labels put on those panels to warn people of the hazards behind.
• Prevent accessible metal parts from becoming "live" by grounding those metal parts through connecting them to an earth terminal. Grounding can be done by connecting to a flexible cable earth wire (for portable equipment), or by means of other permanent earth conductors.

Typical plugs with two or three pins

 

Wiring/ Cords

• Check the flexible cable to see whether an earth wire (green-yellow core) is provided except for double insulated equipment.
• Connection of wires at terminals should be arranged so that they are relieved of excessive strain and cannot be easily pulled off,causing an accident. This is normally achieved by means of appropriate cable clamps.
• Sharp edges over which insulated wires pass, such as openings in sheet metals, can easily cut through the insulation of the wires. Therefore, these edges should be covered with soft insulted materials.

How to select safe electrical products?

 

Plugs and Sockets

• Plugs should conform to the appropriate standards. This has been discussed previously in the Safetywise. Plugs should preferably be of a type that does not require the use of an adaptor. The socket outlets in HKUST are of the 13-Amp square-pin type.
• Main socket-outlets mounted on double insulated (Class II) equipment are designed only for connection of other double insulated equipment.
• Main socket-outlets mounted on Class I (requires protection by grounding) equipment should be provided with safety earth contacts which are reliably connected to safety earth terminals or contacts.

 

Mechanical Strength / Physical Conditions

• The equipment should be designed and constructed with adequate mechanical strength and appropriate capacity to withstand those environmental conditions such as high temperatures, high humidity, rough handling under which it is intended to be used.

 

Essential Markings and Critical Information

• There should be warning symbols on dangerous parts such as live electrical parts, hot parts, which are accessible to maintenance personnel.
• When information is needed for the safe operation of the equipment, this information should be clearly marked on the equipment or given in the instructions for use and supplied with the equipment.
• The safety requirements described above are by no means comprehensive. They are the basic safety features that are more easily identifiable by users without using special instruments or going through sophisticated testing processes. Users are advised to seek help from the Electrical and Mechanical Service Facility (EMSF) or SEPO if they have doubts about the safety condition of their equipment. Electrical equipment meeting the above requirements will provide substantial protection against electric shock hazard.