GB2387488A

GB2387488A - Test probe containing a fuse and a resistor

Info

Publication number: GB2387488A
Application number: GB0208072A
Authority: GB
Inventors: Albert J Duck
Original assignee: AVO INTERNAT Ltd
Current assignee: AVO INTERNAT Ltd
Priority date: 2002-04-08
Filing date: 2002-04-08
Publication date: 2003-10-15
Also published as: GB0208072D0

Abstract

A test probe contains a resistor 6 in parallel to a fuse 4. The resistor is preferably greater than 100k L , may be cylindrical and may be located round the fuse. The fuse may be rated at less than 500mA. A blown fuse will be noted by an unusually low test reading and may be confirmed by an LED or neon across the fuse.

Description

r , FUSED PROBE FOR AN ELECTRICAL TEST INSTRUMENT

Field of the Invention

The present invention concerns a probe for use with an electrical test 5 instrument, particularly for use on electrical power installations and includes instruments such as a multimeter which is capable of measuring voltage, current and resistance. Background to the Invention

10 Multimeters as a minimum have three basic functions, the measurement of voltage, current and resistance. Typically, the user selects whether a measurement of voltage, current or resistance is required by attaching probes to appropriately designated input terminals of the multimeter. In addition, the multimeter will have a dial or switch which allows the user to select a voltage, current or resistance range.

15 If multimeters are used as they are intended, there are no problems, but human errors sometimes occur such as misunderstanding of the circuit being connected to or accidentally selecting the wrong range or function, and hazards can and have arisen. The most likely and foreseeable hazardous fault is in applying a relatively high voltage or energy mains supply across one of the ohms or current 20 ranges. Current ranges are potentially the most hazardous due to their low impedance. For instance, if the leads are accidentally connected to the current terminals rather than the voltage terminals and then the probe applied to a mains voltage, because of the low impedance across the current terminals, a short circuit occurs, particularly if the multimeter is set on a high current range.

25 In order to reduce the risk of such accidents, the UK Health and Safety Executive published a guidance notice GS38, revised in February 1995, "Electrical Test Equipment for use by Electricians". This guidance note provides that all test probes and leads should include a fuse with a low current rating. In the section "Test Probes and Leads", Clause 9 states "(a) the probe (iii) should have suitable high break capacity (hbc) , sometimes known as high rupture capacity (hrc), fuse or fuses, with a low current rating (usually not exceeding 500 milliamps), or a current limiting resistor and fuse.n

The guidance note states that the main risks are that of electrocution, bums and from arc explosion. It also states that the main risk is in testing for the presence of live voltage to see if it is safe to work on circuits. It also gives some working practices. 5 By including a fuse in the probe, if the leads are accidentally connected to the current terminals of the multimeter and then applied to a high voltage circuit, the fuse will blow. The presence of the fuse in the probe also protects against connection between leads such as metal doors of a cabinet cutting through both leads and making a short circuit between them.

10 The main problem with fused probes is that it is not always apparent when a fuse has blown. If the fuse in a probe has blown, and the user is not aware of this and uses the probes to test for the presence of a live voltage, then the user will obtain a reading indicating that no voltage is present. This leads to a risk of electrocution if the user then proceeds to work on a circuit believing it to be safe to do so.

Summary of the Invention

According to the present invention, a probe for an electrical test instrument comprises: a probe body having a probe tip for making electrical contact with a circuit to 2 0 be tested; a fuse located in the probe body; and a resistance connected in parallel with the fuse.

Because the probe has a high value resistor or impedance in parallel with the fuse, in the probe body, if the fuse blows, the probe will still give a reading when 2 5 applied to a circuit where a live voltage is present. The reading will not be accurate, but will be sufficient to indicate that a voltage is present and thereby deter the user from working on the circuit. The resistor/impedance is either a single high integrity type component, or comprises two or more components, such that a single failure cannot cause the probe to become hazardous. The value of the resistor/impedance 3 0 should be sufficiently high to prevent any risk of fire, bum or arc explosion but low enough to make a measurement with an acceptable error to clearly indicate the presence of a dangerous live voltage.

Preferably, the fuse rating is less than 500 milliamps.

Preferably, the value of the resistor is greater than 100kQ

Preferably, the probe body also includes an LED or neon across the fuse. This gives a visual indication when the fuse has blown.

Preferably, the resistor is hollow and the resistor and fuse are arranged concentrically with the fuse inside the resistor. Preferably, the resistor is a cylindrical 5 tubular resistor, and the fuse is positioned inside the tube.

Preferably, the fuse is held in position by resiliently biassed holding means.

This allows for tolerances in the size of the fuse.

Brief Description of the Drawings

o An example of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a cross- section of a probe in accordance with the present invention; and Figure 2 is a exploded perspective of the probe.

Detailed Description

The probe has an outer casing consisting of a first part 1 which contains a probe tip 2 and a second part 3 which houses a fuse 4 and a resistor 5, both of which are housed inside an internal casing 6. The first part 1 and the second part 3 of the 2 o outer casing are connected by a screw thread 7 such that the probe may be easily disassembled to replace the fuse 4. Metal contact portions 8a, 8b provide contact between the fuse 4 and the resistor 5 at each end. A spring material 9 is used in line with the fuse to allow for tolerance in the size of the fuse. The spring material may be a spring or a rubber disc.

2 5 In use, a lead is attached to the end 10 of the probe, the lead being connected to terminals of a multimeter.

In this example, the resistor is 150 kQ rated at 500 V continuously (1.7 W) for high integrity. The clearance distance across its body length is 32 mm which should withstand a 24 kV transient impulse which is well in excess of that required for the 3 0 operating voltage and the expected working installation. If the fuse is ruptured when working at the maximum voltage, the body of the fuse will warm within safe limits, giving a tactile indication that the fuse is ruptured. An LED may be connected across the fuse to give a visual indication that the fuse is ruptured. This example will have an accuracy of 1.5% for meters with a 10 MO input impedance, 15% for meters with

a 1 ME input impedance and for older analogue meters with input impedance of 1 kQN on the 250 V range, 38%.

The test leads may be plugable or fixed.

Claims

1. A probe for an electrical test instrument comprising: a probe body having a probe tip for making electrical contact with a circuit to 5 be tested; a fuse located in the probe body; and a resistance connected in parallel with the fuse.

2. A probe according to claim 1, wherein the fuse rating is less than 500 1 0 milliamps.

3. A probe according to claim 1 or 2, wherein the value of the resistor is greater than 100kQ 15

4. A probe according to any one of the preceding claims, wherein the probe body also includes an LED or neon across the fuse.

5. A probe according to any one of the preceding claims, wherein the resistor is hollow and the resistor and fuse are arranged concentrically with the fuse inside the 2 0 resistor.

6. A probe according to claim 5, wherein the resistor is a cylindrical tubular resistor, and the fuse is positioned inside the tube.

2 5

7. A probe according to any one of the preceding claims, wherein the fuse is held in position by resiliently biassed holding means.

Application No: GB 0208072.9 Examiner: Frederick Fee Clauns searched: I to 7 Date of search: 27 May2002 l Patents Act 1977 Search Report under Section 17 Databases searched: UK Patent Office collections, including GB, EP, WO & US patent specifications, in:

UK C1 (Ed.T): H2E [EAHP] Int Cl (Ed.7): GOlR Other: On-line: WPI, JAPIO, EPODOC Documents considered to be relevant: Category Identity of document and relevant passage Relevant to claims X US 4464623 [CHAMBON] fuse 10, bulb 15, resistor 16, figure 6 I, X FR 2398311 A1 [SINGER] fuse 20, resistor 25, neon 23, figure 3 to X Document indicating lack of novelty or inventive step A Document indicating technological background and/or state of the art.

Y Document indicating lack of inventive step if combined P Document published on or after the declared priority date but before the with one or more other documents of same category. filing date of this invention.

E Patent document published on or after, but with priority date earlier & Member of the same patent family than, the filing date of this application.

An ExecutiveAgency of the Department of Trade and Industly