GB2345550A

GB2345550A - Battery earth leakage detection system

Info

Publication number: GB2345550A
Application number: GB9828774A
Authority: GB
Inventors: Andrew Simon Clegg
Original assignee: Elliott Industries Ltd
Current assignee: Elliott Industries Ltd
Priority date: 1998-12-29
Filing date: 1998-12-29
Publication date: 2000-07-12
Also published as: GB9828774D0; AU1879800A; WO2000039599A1

Abstract

A method or means of detecting earth leakage from a battery 1 comprises measuring the voltage V across the battery 1 when the negative pole is connected to earth, when neither pole is connected to earth and when the positive pole is connected to earth. Then comparing the three voltage measurements and indicating an earth fault if all three are not equal.

Description

2345550 METHOD AND APPARATUS FOR DETECTING EARTH LEAKAGE FROM A BATTERY.

The present invention relates to a method and apparatus for detecting earth leakage from a battery.

Safety whilst working on high voltage batteries is very important. It is very much safer to work on an isolated battery than on one that is earthed. if a battery is earthed it may have a large potential to earth in other places. To avoid this lower - voltage batteries may be earthed in the centre, thus reducing the maximum potential to earth by half. The trend to higher voltage batteries, with higher voltage semiconductors makes this safety feature impossible.

A technician may work on a live floating battery in safety, providing they only make contact with a single point on the battery at any one time and providing that it is totally floating and has not become earthed by some fault or leakage. Unfortunately it is very easy for earth faults to occur; due, for example, to accidental connection with a rack, leakage of electrolyte or deposits on the surface of the battery. To this end many battery systems are installed with a floating battery and an earth fault detector.

A traditional detector circuit is shown in Fig.l. it functions by monitoring current flow through Rc seen as voltage drop M.

Under normal, no fault conditions, current flow (conventional) is from the positive side of the 3S battery to the negative side via Ra & Rb (both are of high value e.g. 500k - 1MO). There is no route for current flow through Rc and therefore no voltage is developed across it. If an earth fault develops at the negative end of the battery (Rf as shown in Fig.

2), current flows through Ra and divides, sharing current through Rb and Rc. The earth fault is detected by monitoring the voltage developed across Rc. The current flow through Rc is inversely proportional to the size of Rf (equivalent -value of fault resistance to earth). If an earth fault develops at the positive end of the battery (Rf as shown in Fig. 3), current flows from the positive of the battery through Ra and Rf returning to the negat-ive end of the battery via both Rc and Rb. The earth fault is detected by monitoring the voltage developed across Rc. The current flow through Rc is inversely proportional to the size of Rf (equivalent value of fault resistance to earth). A device detecting voltage drop across a resistor (Rc) will therefore detect an earth fault, where the earth fault is at (or towards) one end of the battery.

If an earth fault develops in the centre of the battery (Rf), as shown in Fig. 4 there is no potential difference between i) the junction of Ra & Rb and ii) the centre of the battery, therefore no (or very small) current flows through Rf or Rc, and the fault is not detected. It is of course unlikely for a fault to develop dead in the centre of a battery. However due to the nature of batteries the spread of a fault evenly along the battery as shown in Fig.5 is far more common.

Due to the nature of the environment surrounding batteries this fault condition is common where batteries have been installed for some time. The build up of dust and moisture deposits on and around the battery can very quickly cause this condition. Under these circumstances current circulates through Ra & Rb as normal but fault current also circulates through Rf. and returns to the battery via Rf, Once again there is no potential difference between i) the junction of Ra & Rb and ii) the centre of the battery, therefore no (or very small) current flow through Rf2 or Rc. Therefore the fault is not detected.

As can be seen an earth fault at either end of the battery will cause current to flow in resistor Rc. This can be easily detected and this generate an alarm, warning the technician not to attempt to touch the battery. However this circuit arrangement has a major flaw. If the earth fault is symmetrical across the battery (central, equal on both halves, or approximately equal over the whole battery) little or no current flows in resistor Rc and therefore the fault is not detected. This results in a very unsafe battery and worse still, the safety equipment indicating that there is no danger. In addition the arrangement is not fail safe. It is looking for the presence of a voltage. Should a fault condition arise in the detector causing an open circuit in the Rc arm of the resistor array then no fault will be detected. In short the traditional arrangement can indicate an earth fault but it cannot indicate the absence of one.

According to a first aspect of the present invention there is provided a method of detecting earth leakage from a battery, the method comprising the steps of measuring the voltage across the battery, earthing the 4 negative end of the battery and measuring the positive potential with respect to earth, earthing the positive end of the battery and measuring the negative potential with respect to earth, the three measuring steps being carried out in any order, and comparing readings from the three measurements and indicating an earth fault if all three are not equal.

This method offers the benefit over the conventional method in that all types of earth fault are detected as all types of earth fault will cause the measured voltages in either or both of the measurements taken with either end of the battery being earthed to be lower than the voltage measured across the battery.

Also, because the method detects a fault with a change in measured voltage, faults such as open circuits, short circuits and high impedences are detected as faults. The method is therefore failsafe.

The step of earthing either end of the battery could involve connecting the ends of the battery to a reference voltage which is itself referenced to earth. However, it is preferable for the ends of the battery to be directly earthed, for example via a high value resistor.

According to a second aspect of the present invention, there is provided an apparatus for detecting earth leakage from a battery, the apparatus comprising a first connection connectable to the positive terminal of the battery, the first connection leading to a first switch capable of switching the first connection between a voltmeter and earth; a second connection connectable to the negative terminal of the battery, the second connection leading to a second switch capable of switching the second connection between the voltmeter and earth, whereby the voltmeter can measure the potential across the battery, the potential of the positive terminal with respect to earth and the potential of the negative terminal with respect to earth; and means for comparing the three potentials and indicating an earth fault if all three are not equal.

An example of a method and apparatus in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

Figs. 1 - 5 illustrate the method employed by a known earth leakage detector; Figs. 6 - 8 illustrate the method and apparatus of present invention measuring voltages under various circuit conditions without earth leakage; Figs. 9 - 11 illustrate the same conditions with earth leakage; and Figs. 10A and 11A are equivalent circuits to Figs. 10 and 11 respectively.

In Figs. 6 - 10 an apparatus is shown for detecting earth leakage from a battery 1 comprising a number of cells 2. The apparatus comprises an A-D voltmeter.

Two single pole switches 3, 4 each of which can be switched between the voltmeter and a respective large value resistor 5, 6, typically of 5OOkQ connected to earth. The other end of each switch is connected to a respective pick-up wire 7, 8 each of which can be connected to a respective terminal of the battery via 6 a current limiting resistor 9, 10, typically of 51ko.

Fig. 6 illustrates the measurement of the overall battery voltage where the switches 3, 4 connect the battery terminals to the voltmeter. In Fig.7, the switch 4 is switched so as to connect the negative terminal of the battery to earth so that the potential of the positive battery to earth can be measured. In Fig.8, the switches are reversed from their positions in Fig.7, thereby earthing the positive end of the battery and measuring the potential of the negative end of the battery with respect to earth. It will be appreciated, that if the battery is truly floating, then the three measured voltages from Figs. 6 - 8 should be equal.

Figs. 9 - 11 show the operation of the detector for the battery 1 having an earth leakage fault which is represented by an earthed resistor 11, 60% of the way along the battery from the negative end. Fig.9 corresponds to Fig.6 and represents measurement of the overall battery voltage. The resistive connection 11 to earth does not affect the overall voltage because batteries have a very low impedance relative to the earth fault. Therefore, the overall voltage will be correctly read as being the same as in Fig.6. If, for example, the battery 1 consists of five 100v cells, then the measured voltage in Fig.9 will be 500v d.c.

on the other hand, when the negative terminal is earthed as in Fig.10, which corresponds to Fig-7, the meter V will see a voltage significantly lower than the voltage seen in Fig.9. This is demonstrated in the equivalent circuit shown in Fig.10A. This circuit is based on the battery 1 having five 100v cells, the 7 is based on the battery 1 having five 100v cells, the resistors 5, 6 being 500kQ, the resistors 9, 10 being 51kQ and the fault resistant 11 being 2MQ. In this case, the voltage detected at the meter V is 435.2v. 5 Fig. 11 shows the position corresponding to Fig.8 where the positive terminal of the battery is earthed. Again, the meter V sees a lower voltage than in Fig.9. From the equivalent circuit in Fig.11A which is based 1 0 on the same values as Fig.10A, the voltage seen by the meter V can be calculated at 456.8v.

By comparing these voltages and determining that they are unequal, the presence of an earth fault is detected. Even for a fault spread along the length of the battery, although the readings from Figs.10 and 11 will be equal, they will be different from the reading of Fig.9 so that a fault is recognised even under these conditions.

8

Claims

S 1. A method of detecting earth leakage from a battery, the method comprising the steps of measuring the voltage across the battery, earthing the negative end of the battery and measuring the positive potential with respect to earth, earthing the positive end of the battery and measuring the negative potential with respect to earth, the three measuring steps being carried out in any order, and comparing readings from the three measurements and indicating an earth fault if all three are not equal.
2. An apparatus for detecting earth leakage from a battery, the apparatus comprising a first connection connectable to the positive terminal of the battery, the first connection leading to a first switch capable of switching the first connection between a voltmeter and earth; a second connection connectable to the negative terminal of the battery, the second connection leading to a second switch capable of switching the second connection between the voltmeter and earth, whereby the voltmeter can meaure the potential across the battery, the potential of the positive terminal with respect to earth and the potential of the negative terminal with respect to earth; and means for comparing the three potentials and indicating an earth fault if all three are not equal.