Archive for the ‘Electrical safety’ Category

Electrical safety checks.

These checks are designed for technicians in developing countries who have little or no test equipment, they are very basic checks that might save a life. If you want to know more click the link at the end of the article.

Electrical equipment should have safety checks done at regular intervals, once a year should be enough or if you have done any serious work on the electrics of an appliance. This is something that requires a keen and well-trained person to do it, it would be very easy to skip through the checks and not do them properly, but this is a very dangerous practice. Electricity is very dangerous, and whilst a fault on an anaesthetic machine such as a leak can be dangerous it is seldom a fault that occurs suddenly with serious consequences. With electricity, a faulty earth lead, for example, may pose no danger for a long time so long as the rest of the wires are in good condition, but should the live wire become loose and touch some part that it shouldn’t, the next person to use the machine could be killed in an instant.

For this testing you should have an electrical safety checker, it is a package of electronics into which you plug the machine under test, then by operating a rotary switch the machine is put through a number of checks. This piece of test equipment will cost a lot of money and your hospital may not be able to afford it, this is no reason not to do the checks.

There are many checks that can be done with a multi-meter, your eyes and common sense. Around 80% of all faults are found by visual inspection. Electrical safety can lead you into the world of amps, microamps, milliamps, ohms and milliohms, earth leakage currents, of floating connections and earth bonding. Fortunately, one of the things most likely to save your life or that of a patient or other member of staff is one of the easiest to check for and needs little equipment to do so.

A good earth connection.

First a simple look at the main classes of electrical equipment.

All electrical equipment can be put into two classes, class 1, class 2 and class 3.

Class 1;
Any equipment that has exposed metal parts. Parts that could become live and therefore dangerous in the event of an internal problem. This type of equipment always needs an earth. For example, most humidifiers with a heating plate.

Class 2;
Screenshot 2018-12-04 at 09.56.55
Any equipment that is double insulated, that is it is made in such a way or of a non conductive material like plastic, that no matter what happens inside you should never be at risk of an electric shock. For example, a domestic hair dryer. Will equipment be labelled with its class? Probably not, though new equipment might.

Class 3
Class III equipment is low-voltage equipment. The voltage is so low that a person who gets in contact with it does not get an electric shock.

The equipment runs either on battery or on an external power supply which creates a supply voltage of less than 50 VAC. The testing of Class III equipment is done in conjunction with the power supply tested in Class I or Class II.

The equipment classes define the method of protection against electric shock. For household appliances this is good enough but not for medical equipment.

Medical equipment of one class can be used without a connection to a human body (e.g. suction pump), with a patient connection (e.g. pulse oximeter) and inside the body of a patient (e.g. electrosurgery unit). That is why the appliance classes are again divided into different types. The types define the degree of protection.

For this reason we find two symbols on the rating plates of medical equipment, one for the method of protection (class) and one for the degree of protection (type).

These are the types.
In combination with medical equipment of Class I, II, III.
Type B
BB symbol.
The standard degree of protection against electric shock. No electrical contact with a patient. Equipment may be connected to the earth. Patient connections are not conductive and can be immediately released from the patient. Standard values for permitted leakage currents are demanded which are stated under the respective test procedure.

Type BF.
BFBF symbol
Equipment is safe for electrical connection to the patient but not directly to the heart. The patient part of the equipment is isolated (floating circuit) and has to be separated from earth. Standard values for permitted leakage currents are demanded which are stated under the respective test procedure.

When the equipment can be used in combination with a defibrillator this symbol has to be printed on the rating plate. It means defibrillation-proof.
BF defibBF symbol with a defibrillator.

Type CF
Equipment provides highest degree of protection against electric shock. It is safe for electrical connection to the heart of the patient. The patient part of the equipment is also isolated (floating circuit) and is separated from earth like BF. The permitted leakage current is much lower than for type B and BF. The values are stated under the respective test procedure.

When the equipment can be used in combination with a defibrillator this symbol has to be printed on the rating plate. It means defibrillation-proof.
CF defibCF symbol with a defibrillator.

Class 1 equipment may have the symbol shown above to indicate an earth. There is an agreed class 2 symbol and it is a square within a square as shown above.
So, how can you tell one class from another?

Here we are considering the type of plug found in the U.K. Three brass pins with the middle one being the earth pin.

Class 1 equipment will have an earth, which needs a 3-core mains cable and a 3-pin plug. Equipment that looks like it is made of metal or has some metal that you can touch.

Class 2 equipment will have a 2-core mains cable and a 2-pin plug. However, some class 2 appliances may have a 3-core mains cable and a 3-pin plug for the convenience of the makers, (they plug into a normal 3-pin socket without a converter). A give away can be that the middle pin (the earth) on the plug is made of plastic though at first sight it may look just like a normal plug.

Two important rules:

1. Always unplug an appliance before checking it.

2. If you take the covers off, be very careful what you touch, capacitors and the back of TV type screens can hold a lot of charge.

With no test equipment, how do you check for electrical safety?

Observation and a medium bladed screwdriver. You can use the ubiquitous Swiss army knife or a multi-tool but remember………. they are not insulated. For advanced testing use a torch battery some wire and a bicycle light bulb.


The easiest sort of battery to use is the 9-volt type with two metal contacts side by side. If when you buy this type of battery in proper electrical shops, you should also be able to get the correct connection, two correctly shaped contacts with a plastic cover and two wires coming out. See photo above.

What am I looking for? Unplug your equipment first.

1. Check the mains cable and plug. Check the cable for damage, cuts, splits, burns and exposed metal conductors. Check the body of the plug for damage.

2.Check the plug is correctly and tidily wired (no stray wires), Check that the fuse is a sensible size.

If you are not sure what it should be, if the maker’s plate tells you the wattage, divide that by the local voltage and it gives you amperage used. For example a 100-watt heater divided by the local voltage of say 220volts gives you 0.45 of an ampere, in this case you would probably fit a 1-ampere fuse. If your appliance has its own fuse holder, fit a fuse as calculated and then in the plug fit a higher fuse with a maximum of 13 amperes. If the appliance has no fuse holder then fit the calculated fuse into the plug.

3. If you are able to and feel the need to do look inside, check for the signs and smells of burning. Clean the insides of rubbish and dead insects, make sure that any air vents are clear and that cooling fans are working.

If the appliance has an earth check that it screwed to something metal. If necessary, unscrew it and make sure it is touching the metal and not a coat of paint.

Using your battery and bulb. Connect a piece of wire to the metal side of the bulb and from there to the positive (+) contact on the battery. Connect the second piece of wire to the negative (-) contact on the battery. This will leave you with a free end of wire.see photo above.

Before you start check your test equipment works. Touch the free end of the wire to the contact on the bulb, the bulb should light. To test the earthing on a piece of class 1 equipment, connect the free end of the wire to the earth pin on the plug (the one in the middle), touch the contact on the bulb to the metal parts of the equipment under test. If there is an electrical path from the earth pin through the mains cable to the metal body of the equipment, the bulb will light up. If it does not light up there is a problem with the earth, either it is not there or it is a very high resistance. Investigate the inside of the plug, and if you feel happy to do so, the inside of the equipment. Think about the matter logically and try and find where the earth path has got broken, but if in any doubt try and find an electrician in the local town.

As a bonus, you can use this arrangement to test fuses and other light bulbs like those in your laryngoscope or ophthalmoscope.

What can be done to correct faults? Unplug your equipment first.

Once again I can only repeat that the best advice is to consult an electrician. However, in extremis, you may have to do some repairs yourself. Damaged mains cables or other wires can be covered, as a temporary measure, with insulating tape or sleek type plastic tape. If a copper conductor is showing in a cable, make sure that there are not two conductors bare and touching each other. If you think that the bare conductors are touching or might be able to touch together do not use the appliance until the cable is repaired. If a conductor is showing insulating tape may be used to get you through the list, take care not to get the damaged part wet, but after the case is over then the cable should be replaced with a new one.

Detachable mains cables can be swapped for a good one. Plugs can be re-wired.

Check the wall sockets in a room.

If you are able to get the electricity to the room shut off do so. If not do this carefully, remove the socket cover and look inside, make sure that all the wires are connected.
Do not touch anything inside.

The colours of conductors can vary around the world and with the age of the equipment. Make a note of which colours go where before you take them apart. Earth is always green or green with a yellow stripe. Replace fuses with the calculated value or the lowest value you can find.

What is happening if something goes wrong?

If all is well. Earth is connected to the metal case. Electricity comes into the equipment, lights up the lamp and goes out again via the neutral wire.

If things go wrong but you have a good earth connection. Earth is connected to the metal case. The live wire has come loose and is touching the metal case. You switch on; the electricity goes from the live to the metal case and safely to earth rather than through you because there is less resistance. Also, if you have a sensibly low rated fuse that will blow cutting off the electricity. You may still get some sort of shock but hopefully the greater part will go to earth. Do not depend upon a fuse to blow at exactly the stated value.

If all is not well. If things go wrong and you have no earth connection. With the live wire touching the case inside, you turn the equipment on. The only place the electricity can go is through you, if you are touching the metal case, and to earth, possibly stopping your heart on the way. The fuse may or may not blow but in any event is unlikely to do so quickly enough to save your life.

Remember three things:

1. Most towns will have an electrician of some sort. If in doubt or you feel out of your depth, try to get hold of one to help you.

2. A good earth could save your life.

3. If there is any doubt and no help is available, do not use the equipment.

Two cheap and useful tools.

Socket tester.

This particular one is used in British type three-pin sockets, I guess they are available for other socket configurations. Plug it into the socket and the LEDs will illuminate indicating if there is a fault or if all is correct. It also makes a noise. They are also useful when you are working on electrical circuits, plug it in and it will make a noise if there is power on the socket. You can then go to the fuse box and take out the fuse or flick the trip off and you will know if you have removed the power from the circuit you want to work on because the noise will stop. Leave it plugged into another socket on the same circuit while you are working on the circuit if someone turns on the power without telling you it will sound to warn you.

It can indicate seventeen different wiring fault conditions. There are others that will show fewer faults conditions.


Non-contact voltage detector.

This will detect any AC voltage between 90 and 600. The tip will illuminate and it will sound if electricity is detected when the tip is close to a wire, socket or a switch.


An excellent article on electrical safety can be found at Frank’s hospital workshop,  it covers it in more detail. Click the link below.

Frank’s hospital workshop, electrical safety.

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