GB2235982A - Earth continuity checking system - Google Patents

Abstract

In an earth continuity checking system of a mains power supply, a transmitter (4) located at the fuse box or point of known good earth injects signals onto the earth and neutral wires (3, 2) or earth and live wires (3, 1) and receivers (5) are situated at points where earth continuity is to be checked. The receivers can be fixed or mobile installations and can be used to continuously monitor the earth line and to disconnect live and neutral via an isolator (6) in the event of loss of earth continuity. Loss of neutral continuity and short circuit neutral to earth are also checked with this system. <IMAGE>

Description

EARTH CONTINUITY CHECKING SYSTEM This invention relates to a signalling system that detects the loss of mains earth to an electrical installation.

Electrical systems are in use worldwide which utilise a protective mains earth wire to conduct electricity during a fault condition. The integrity of this wire is infrequently checked and is often only known to be good when a fault has caused a fuse to blow. This invention continuously monitors the earth conductor and signals to the user that the earth is faulty, giving warning prior to a dangerous fault condition occuring.

The invention has developed in relation to communication systems devised for mains borne or DC power line signalling but specifically addresses the problem of a broken earth wire situation and its detection.

Particular attention to British Standards 6396 has been made with the view to meeting the earth integrity monitoring requirements for office furniture and office screens. This specification highlights the dangers of using multiple sockets and extensions in a typical office environment.

Systems are already well known that measure the current in the earth conductor and shut power off if the current exceeds a threshold.

Clearly if the earth continuity is broken then such equipment will not shut down based on current and must therefore revert to voltage measurement of earth to detect a fault condition. This voltage measurement has to be referenced to some point. This point is sometimes the neutral line which in itself is prone to voltage transients due to load switching leading to false detection switching. Sometimes a seperate reference earth point wire is used but involves special wiring routing local to an installation. This latter system is generally restricted to local bench or desk installations.Systems are also known which inject signals into the earth line and measure the resistivity and hence effectiveness of the earth, but restrictions on how many of these systems can operate in close proximity exist because of a build up in earth currents and generally a detection system at every outlet would not be practical with such a system.

Even so, these systems do not confirm that earth is continuous back to the fuse box and it is this requirement that is solved by this invention.

According to the present invention and with reference to Fig.

1, there is a transmitter unit 4 situated at the fuse box or a point of known good earth and a plurality of receiver units 5 situated at points where earth continuity is to be checked.

These can be fixed or mobile installations depending on the requirements but fixed is generally preferred to provide continuous protection. These units are connected via Live 1, Neutral 2 and Earth 3 conductors which are present in electrical installations.

The transmitter unit uses a well known technique in mains borne signalling of transmitting a carrier signal onto the wiring via a suitable isolating transformer and capacitor at a frequency and power level to conform to local regulations.

This carrier frequency can be modulated in frequency or amplitude to form a data communications link for high integrity signalling systems or can remain as carrier only to reduce system complexity and cost.

This signal is injected onto the neutral and earth conductors via 10 and 11 and will radiate throughout an electrical installation including all equipments and extensions plugged into the installation. Alternatively, the signal could be injected between live and earth, but the neutral method is preferred because it checks the loss of neutral as well.

Receivers distributed throughout the installation will pick up this transmitted signal via 12 and 13 and deduce that a connection back to the transmitter must exist for the signal to get through and if the connection to the transmitter is present, then so must the connection to earth.

Loss of the signal may be from 3 sources.

Firstly, the earth wire is broken and whilst all equipment connected to live and neutral is still operating it is not safe to do so. The reciever therefore, can signal a fault or shut the installation down via isolator 6 depending on the requirements of the installation.

Secondly, the neutral wire is broken and this will cause a supply failure situation whereby none of the equipment will operate and the user would seek help to restore operation of equipment. Loss of neutral is still a dangerous situation since live is still connected to the equipment and if the installation required a complete shutdown via isolator 6 can occur.

Thirdly, a short from neutral to earth would cause loss of signal and whilst equipment would still operate safely, earth currents could be excessive and present an unchecked danger.

The advantage of using this invention is that of continuous checking back to the fuse box or known earth is being performed and any break condition will be detected before a dangerous short to earth on equipment occurs, thereby reducing the risk to users of equipment.

A practical embodiment of a transmitter is shown in figure 2 which shows a simple form of carrier only arrangement.

A simple power supply 4 derives power from mains live 1 with respect to Neutral 2 via a transformer/diode bridge circuit to give a DC output to an oscillator 5 which continuously drives a buffer 6 which drives a tuned transformer 7 and capacitor 8 circuit. This exitation of the primary causes a secondary voltage signal to appear on the secondary of the transformer which via a capacitor 9 passes on to the earth line with respect to the neutral. This signal then radiates by conduction through the earth/neutral wiring system 2 and 3.

A practical embodiment of a receiver with outlet shutdown is shown in figure 3. A PSU 4 provides power to a carrier detect circuit 5 which receives a carrier signal from the earth/neutral connection via capacitor 6 which passes the signal to a transformer 7 tuned by capacitor 8. This signal passes into the carrier detector via capacitor 10 and the carrier detector output drives a control line 9 which energises an isolator 11 which has two poles to isolate live 1 and neutral 2 to the users outlet point 12 and 13.

A 'No Carrier' situation will de-energise the isolator and therfore remove the users power rendering a safe condition.

Clearly the carrier detect can operate many functions for example, warning lamps and audible devices to alert the user to a condition which requires attention.

In a practical installation which uses recievers at all outlets it is conceivable that the detection of an earth break situation could be located by taking note of which detectors are indicating a fault and which are not and deducing from the wire routing which section of cable must be faulty.

Claims (1)

1) An earth continuity checking system which comprises of a transmitter and receiver (s) signalling system, using mains earth and neutral wiring or mains earth and live wiring as the medium for signal paths, whereby the transmitter is located at a point of known good mains earth and the receiver (s) is situated at a point where continuity to that known good mains earth point, is to be checked.

2) An earth continuity checking system as in Claim 1 which is continuously monitoring the earth continuity prior to a fault occurring 3) An earth continuity checking system which as in Claim 2 can act in a manner to make safe an electrical installation which has developed a break in the earth connection, by de energising an isolator.

4) An earth continuity checking system which as in Claim 3 can also check for short circuit between neutral and earth wires.

5) An earth continuity checking system which as in Claim 3 can also check for an open circuit neutral wire.

6) An earth continuity system which as in Claim 2 can pinpoint a break in earth wiring if a plurality of receivers are in use.