Hi Sefton, this topic crops up regularly because the manufacturers view seems on face of it at odds with GS38. The manufacturers say that they implement sophisticated internal input protection measures on their MFTs that exceeds GS38. If inline fuses in the leads are used this adds extra error terms to any measurement, however, most manufacturers offer GS38 inline fused leads as an aftermarket accessory for those who must have them. These additional error terms could be compensated for or eliminated entirely by implementing a four terminal test method and the use of special Kelvin leads, the manufacturers argue this would nearly double the cost of an MFT and believe there is little market appetite for this solution. Below I have added Emails I have received from Fluke and Megger after asking about this issue:
[FONT=&]Fluke’s view on GS38 fused leads:[/FONT]
[FONT=&]Hi Mark,[/FONT]
[FONT=&]It is Fluke's opinion that in applications using Fluke name brand instruments, fused test probes provide little if any added protection. Fluke's primary concern is that fused test probes may result in serious shock hazard due to a false voltage reading. A more effective solution is a meter properly designed to handle overload conditions. For the Amp and milliamp functions, Fluke meters incorporate a specifically designed and tested fuse that is effective to 1000 Volts as the full breaking capacity of the system which limits the energy in a manner that is as effective as a fuse located within a test probe.
It is additionally Fluke's position that a fused test probe is potentially more hazardous when used with any DMM or MFT for the following reason. It is estimated that over 90% of most electrician measurements use the AC VOLTS functions of a meter. Current measurements are generally less than 10%. If the fuse in the test probe is damaged and not noticed or properly corrected, then false voltage measurements will occur which potentially create a shock hazard for the user. [/FONT]
[FONT=&]For Fluke meters, a damaged fuse within the amps function of the meter does not affect voltage measurements, so the potential of shock hazard due to false readings is eliminated. Note that for the other meter functions such as ohms, Fluke meters are designed to prevent hazard for overload conditions equivalent to the maximum AC volts input as stated on the meter. These functions limit the energy by impedance. This opinion on fused test leads is limited to, and only applies to their use when used with Fluke brand test instruments.[/FONT]
Megger’s view on GS38 fused leads.
Hello Mark,
Ref: Fused leads
Only a few sections in GS38 refer to fused test leads. They are 7b, 9.iii, 12a, 12b and 18. Only 7b states a justification for the test lead fusing.
7b - refers to instruments that are unsafe on some ranges for connection to high energy supplies
9iii - is a recommendation for fused leads but with no referenced justification
12a & b - refers to voltage detection devices, not multi-function instruments
18 - recommends fused leads when voltmeters and particularly multimeters are used.
Fundamentally, GS38 recommends:
"The test probes and leads used in conjunction with a voltmeter, multimeter, electrician's test lamp or voltage indicator should be selected to prevent Danger. Good test probes...will have the following:".
However, when considering fused leads, GS38 does not recognise the huge amount of design for safety that modern professional grade instruments now contain. Much of GS38 is based on too much exposed metal (probe tips) creating short circuits or protection against instruments, and especially multimeters that could be dangerous if used on the wrong range without properly designed protection, such as accidental current range selection.
Instruments such as the Megger MFT1500 series provide live circuit detection when on continuity and insulation ranges as well as the live testing functions. These functions work even if the internal fuse has failed (or been removed). Now if a fused lead set is used which has had the fuse removed (or the fuse has failed whilst in use) the instrument can no longer provide the live circuit detection. Consequently a live circuit could appear to be dead, which would not happen with a non-fused lead.
Why would a test lead fuse fail in use? Take loop impedance testing. This is a live supply test, during which most loop impedance testers use a high current in the phase to neutral circuit, which would rupture a 500mA fuse, the maximum size recommended by GS38. Protection against potential short circuits to earthed metalwork due to damaged leads is a common argument for the use of fused leads but if implementing this approach, the limitations need to be understood.
Clearly there are many factors to be considered when deciding whether or not fused leads are appropriate for the proposed testing. Consequently Megger always recommend a risk assessment prior to choosing which type of lead set to use. This needs to take into consideration; the category rating of the installation, the type of testing, the safety features of the instrument, the potential risks from damaged test leads and the limitations and hazards associated with different lead types.
Regarding the colour of the test leads it is not a requirement for the leads to be in harmonised colours.
Regards
Megger TSG
Despite all this I still prefer to use GS38 fused leads with any MFT that I use, more for my personal safety than for protection of the instrument. Having witnessed an shorted tester, resulting arc flash and the injury to good friend, I want all the PPE and protection I can get. I'll leave you to make a judgement from the view of myself and others on the forum.
