Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Patent
1990-03-21
1992-09-29
Skudy, R.
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
H02H 318, H02H 326, H02H 342
Patent
active
051518414
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an electrical safety apparatus, and particularly, but not exclusively, to apparatus for protecting against electric shock which may occur due to contact with electrical conductors connected to a power source.
It is known to provide earth leakage circuit breakers (elcb's) arranged to disconnect electrical conductors from a power source in response to the detection of a leakage current to earth. This prevents electrical shock to a user who, for example, accidently comes into contact with the live conductor of a cable connected to a mains electrical power supply.
However, elcb's do not provide protection in the case where there is no current to earth. For example, if a user comes into contact with both the live and neutral conductors of a cable connected to a mains power supply there may be no current to earth, and the elcb will provide no guard against shock to the user.
The electrical characteristics of the human body are generally different to the characteristics of standard electric appliances, such as for example, lawn mowers which need to be driven by a domestic mains power supply. For example, the IEC publication 479-1 chapter 1, states that the impedance of the human body varies with touch voltage, and gives a table of body impedance for values of touch voltage from which it can be seen that at 25V or below the minimum expected impedance for 95% of the population would be 1750 ohms and is generally much higher. On the other hand, domestic electrical appliances are likely to have an impedance at this range of voltage of less than 600 ohms.
In the case where a circuit breaker employs a relay to connect and disconnect an electrical power source and a load (appliance), it is necessary for the relay to operate each time the appliance in use is switched on and consequently must withstand accidental short circuit conditions when energised. The relay must have minimal contact bounce and must not be subject to momentary contact opening and closing due to heavy short circuit currents causing the contact blades to move and open their contact points, leading to the possibility of contact welding. It is therefore important for circuit breakers to have adequate short circuit protection.
A power supply system comprising an a.c. voltage supply source, a load and a connecting cable between the source and the load is disclosed in European Patent 096915. One feature of the system is the use of a parallel arrangement of at least one capacitator and a direct current (d.c.) source placed in a d.c. circuit comprising the two cable conductors of the connecting cable. When the connecting cable is disconnected from the load (open circuit) the voltage across the capacitator increases -- when a threshold circuit determines that a threshold voltage has been reached it causes the a.c. voltage supply source to be disconnected from the connecting cable. In a second feature if, after the connecting cable has been disconnected from the load, a load of low impedance (short circuit) is connected (accidentally) across the cable conductors, there is provided a system for preventing the full a.c. voltage supply (700V) being connected to the connecting cable: the power source attempts to power up to a voltage of 30V a.c. If the impedance of the load is high, the voltage across the output exceeds a 20V threshold value and a long as the direct current path across the capacitor referred to in the description of the first feature is closed then the full voltage supply (700) is applied to the load. If the 20V threshold voltage is not exceeded due to the presence of a short circuit or load of very low impedance, then the power source does not supply the full (700V) voltage to the output.
The present invention provides electrical safety apparatus arranged to be connected between an electrical power source and a load, said apparatus comprising means operative before the normal electric power supply is applied to the load to apply a test voltage or current to the load, and means arranged to prevent the application of
REFERENCES:
patent: 2789255 (1957-04-01), Mekler
patent: 4523248 (1985-06-01), Schmale et al.
patent: 4630161 (1986-12-01), Seitz
B & R Electrical PLC
Skudy R.
To E.
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