Electricity queries
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Power tools and earthing With reference to the July 2011 edition of The Home Handyman, the article ‘Room for Improvement’ states: “all electrical appliances sold in South Africa must be fitted with an earth wire”.
In 2007 I bought a set of Ryobi power tools – power drill, orbital sander, jigsaw and angle grinder, and all of these came fitted with two-pin plugs. Obviously no earth wires are fitted to these appliances. Should I be concerned about the safety of using these appliances? Dave Aitken, Kloof
James Reynolds, our electrical expert, replies: Section 5.1.3.2 of the SANS 10142 states that people, animals and property shall be protected against harmful earth fault currents by protective measures such as:a) Earthing and bondingb) Electrical separation of circuitsc) The use of an isolating transformer with an output of 50 volts or less, ord) The use of electrical equipment that is double insulated.You will find that all portable electrical appliances which do not come with an earth connection will fall into the 'double insulated' category.
On the appliance will be stamped a symbol (one square inside another) indicating that an appliance is double insulated.For an appliance to be deemed 'double insulated', no external metal parts may be exposed to the electrical circuit of that appliance. This is achieved by manufacturing non-conductive parts which form a barrier to external metal parts.
An example of this is putting nylon gears between the motor and the chuck of an electric drill, or a nylon boss between a mower blade and motor shaft.If you cut into the electrical cable of these appliances while energised, the earth leakage unit in your distribution box will operate once it senses 30 milliamps or more going to earth, which will happen if you come into contact with the fault, or the individual circuit breaker will trip, sensing a dead short between the 'live' and 'neutral' conductors.Another quick check is the SABS stamp on these appliances. This indicates that the appliance has been tested and deemed safe by the controlling authority. |
An earth leakage
Can you assist with an electrical problem? The earth leakage circuit breaker for a three-phase trips and continues to trip instantaneously when a breaker feeding five plugs is off or on, no load. If there is a load on this breaker, earth leakage stays and trips in its own time.
Eddie Kyereh Baffour, Johannesburg
James Reynolds, our electrical expert, replies:
In dealing with domestic electrical installations we sometimes require a three-phase connection. Either the collective single-phase load requires this, or the consumer has three-phase equipment. The advantage of three-phase design is compatibility. A three-phase 100kW motor is substantially smaller than an equivalent watt single-phase motor.
Now, with the complexities of a three-phase load, there are numerous differently designed earth leakage units (ELU). These ELUs have what is known as different operating graphs. Simply put, they operate at different speeds and loads. So firstly, the ELU must be suitably compatible with the load it is required to protect. If you were to take the specs provided by the manufacturer of your three-phase appliance and go onto any circuit breaker manufacturers’ website, they would advise and supply the required ELU compatible to your load. In your instance, we need to understand what the ELU is doing or required to do.
Ninety-nine percent of all three-phase earth leakage units, when operating, open (switch off) all three phases. The ELU operates when it detects an imbalance between the current flowing to the load and the return current. This imbalance occurs when there is a flow of current to earth. So, if the ELU was not operating (tripping) for a substantial period of time and then all of a sudden started operating, there has to be a problem.
The first, and most likely, possibility is that you have a high resistance fault to earth. The fact that it operates on no-load tells me it has to be between earth and the neutral conductor. You get a certain amount of no-load voltage in the neutral created by other loads on the same circuit. To eliminate this, ensure that the three-phase ELU only protects a three-phase load. You also have a star point in some three-phase loads, which exposes this problem more frequently.
If this problem occurs on a three-phase load that does not have a neutral connection, then you have a high resistance fault to earth. Remember, current will flow along the path with least resistance. So, while the load is being operated, current will flow between the three phases, but once you switch off the load, then the current diverts to the fault path. Any electrician with a resistance tester will be able to confirm this.
Secondly, you could have a problem with the ELU itself. It is common to find that after a while, these units deteriorate due to excessive loads or because they are badly matched with the loads they protect. Some loads have high start-up currents, which puts extra strain on the ELUs.
Pump woes
Can one of your experts can assist me with the following matter: My neighbour, who is a potter, recently gave me a type of pump which she inherited from her father and which, she said, does not work.
The particulars obtained from the body of the pump are as follows: Model VS-651 – 115 volts – 0.5 amps – 60 cycles AC only. Manufactured under Burgess Patent by Burgess Vibrocrafters Inc., Gaylsake, Illinois, USA. The glass body is marked off as follows: 16 oz, 12 oz, 8 oz and 4 oz.
What worries me is that my neighbour said that when she connected the pump to a power outlet, the pump made a “clattering noise”! I believe I am correct when I say that the machine must not be directly connected to the local 220/240 volt AC system and that one must use a 240-120 volt (step down) transformer when using the pump.
Cecil Rosser
James Reynold, our electrical expert, replies:
Burgess Vibrocrafters Inc. build motors and pumps for the huge American appliance market. There are over a hundred different manufacturers using their VS-651; from jigsaws to spray-painting, washing machines and sewerage pumps.
With the little information I have on this specific pump I can only comment on the manufacturer’s technical specifications. This is the information supplied on the attached plate.
Firstly, the voltage of 115V is standard for all American appliances. This requires you to install a step down 220 to 110V transformer for it to be used locally, as mentioned by you.
The frequency is 60Hz (cycles) whereas we use 50Hz. This will have an influence on the motor speed, making the pump less efficient
The most telling information is the 0.5 amps. The output power is 115V x 0.5 amps = 67.5W. This is the same energy used by a standard incandescent globe. With this power you can run a fish tank pump, which has virtually no head (difference in height between inlet and outlet).
If, as you state, it has been connected directly to 240V for a short period of time, I would not be surprised if the pump still works. The knocking noise could be the telltale sign that this pump is driven by a solenoid motor. A solenoid motor is a coil with a bar in the centre of it. As the current changes phases so the bar gets shunted backwards and forwards 50 or 60 times a second depending on the input frequency.
Most of the electrical components will be able to deal with the increase in voltage. But, as for the mechanical pump, I would expect some physical damage. Firstly, the seals being as old as they are would perish instantly due to the excessive force, and secondly, any gearing would not be designed for the force applied to it by the motor.
For further information, contact James on 031-311-9048.
