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Posted By Topic: New Megger or Multifunction?

jonmac
Nov 03 2018 09:53

Hi there. I am an electrician who has been out of the trade and off the tools for the last 20 odd years. I have during this time kept my practising license up to date which I have used for occasional jobs for family and friends. I am now semi retired and now have a job to do for my brother to wire his new boat shed. I will also have a job to wire a mates new house later neext year. I decided to check all my tools including my megger which is a battery unit thats about 40 years old now. On checking the calibration I found that the voltage output with a 1 megohm resistor connected is about 16% below that of the open circuit voltage. The standard calls for a maximum of 10%. This has prompted me to look at either replacing the megger or buy a multifunction device. I believe that I can do all the tests required to meet code requirements with just a good megger and multimeter. Your opinions on if its worth stumping up the extra dollars to get a multifunction device would be appreciated.
   

pluto
Nov 03 2018 10:11

use the meggar you already have, how you measure thr output voltage, did you use an old voltmeter or a high impedance voltage?

The the only additional test equipment required is a EFLI meter it enables easy testing of polarity and earth cont., for socket outlets. No need for an RCD tester fpr new work.
   

DougP
Nov 03 2018 10:20

I would go for the multifunction.
Some people prefer separate instruments, but I haven\'t found it a problem being all in one (with a separate multimeter & clamp meter).

I went for the Fluke 1600 series which were reasonably priced, (around $1400 when I bought it), but are now over $2k.

The Fluke works well, is easy to use, provides all the necessary functions and has great battery life.

The loop impedance testing is invaluable for faultfinding, and as it has a voltmeter function as well (some multifunction meters don\'t), it\'s usually the first piece of equipment I grab for diagnosing problems.

I have heard from others that the KEW/Kyoritsu 1610B is good for the price, but it doesn\'t have the same functionality as the Fluke.
   

AlecK
Nov 03 2018 10:31

I think you\'ll find that that 10% figure is only in a Note in \"3017\". It isn\'t a requirement. In fact none of \"3017\" is a requirement.
So as long as your instrument meets the requirement set by Section 8 of \"3000\" it\'ll be fine. All it has to do is provide 500 V dc when connected across 1 megohm. If it can\'t. then start by changing the batteries.

If you do need to buy a new meter; I wouldn\'t be looking at MFTs unless I had ongoing work to justify the investment.

   

jonmac
Nov 03 2018 13:09

Thanks for the prompt replies Guys. I tested the megger with my fluke 75 multimeter. The voltage output with a 1 Megohm resister connected was 449 volts vs 524 volts open circuit. The batteries were new so the meter is not meeting the 500v test threshold as noted by AlecK. Maybe the tolerances of the older battery megger instruments is not as close as what we have come to expect from modern instruments of today.
For the loop impedance testing I can still do this with a manual test and calculation so I am tending towards a replacement megger unless there is a strong argument for a multifunction tester.

   

mowgli
Nov 03 2018 17:10

Jonmac what do you use for checking main earthing conductors? Most multimeters won\'t cut it as their test current is too low.
   

jonmac
Nov 03 2018 20:00

Hi Mowgli. I have been using the megger on the ohms scale for such tests.
   

mowgli
Nov 03 2018 20:07

Jonmac perfect
   

AlecK
Nov 04 2018 09:38

\"Most multimeters won\'t cut it as their test current is too low. \"
Right answer, but wrong reason.

Not directly related to OP, but worth an explanation.

The reason a DMM isn\'t suitable for earth continuity measurements is not about the test current, it\'s about the inherent error (that ALL meters have) being as big or even bigger than the value being measured.

For analogue, we need a meter with a low-ohm scale (max 3 ohm full scale deflection) so that the error, which is a % of FSD, isn\'t significant compared to the reading.

For a DMM, the error will be expressed as a % of reading and a number of \"digits\"; the \"digits\" being the least significant digit of the reading for the scale used. For a good DMM, the error will be 3 digits; shitty meters could be 4 or 5 digits. On ohms, most DMMs read to 0.1 ohm, so the \"digits\" part of the error will be +/- up to 0.3 Ohm. That\'s in same ball-park as the values we are trying to read; making the results unreliable at best.
The resistance scales on most digital IR meters read to 2 decimal places, so the \"digits\" error is now only +/- up to 0.03 ohm - giving acceptable level of accuracy. The analogue ones generally have a 2 or 3 ohm scale, so a few % of the FSD is an acceptable error factor.

Some DMMs also have suitable scales. But what really does NOT work is using one of those \"Fluke\" DMMs with the fork on one end, that a lot of apprentices were issued with a few years back. Their ohms scale only reads to whole ohms, with either a 3 or 4 digit error. Trying to measure fractions of ohms with an error level of +/- several ohms is simply a waste of time.

The big problem with digital instruments is not the actual meters but the human tendency to believe that the reading we get is somehow \"more accurate\" just because it\'s digital.
   

daniel2
Nov 04 2018 11:08

OP, how long are you going to be in the trade for? Megger test equipment are brilliant but pricey. Maybe have a look on TradeMe for a second hand one.

Kyoritsu (Japanese) is a good, cheaper option.
   

evanh
Nov 04 2018 11:09

I\'m happy with mowgli\'s explanation. Of course accuracy is the real reason, but the way the low-ohm accuracy is achieved is through a substantial increase in the measuring current.

This higher current requires more expensive meter design so is only put in when it\'s a demanded feature.

It\'s just another way of saying the same thing: - The meter has to be designed for the job.


   

mowgli
Nov 04 2018 14:28

Some good info about low ohms resistance testing in this topic.

Personnaly I think the resolution/accuracy requirement goes without saying. You wouldn\'t measure the width of a hair with a meter rule. Neither would you use a tape to measure your commute to work.
https://www.electricalforum.co.nz/index.php?action=more_details&id=1515458330
   

jonmac
Nov 09 2018 16:57

Guys, thanks for all the advice. I have managed to pick up a second hand Kyoritsu 6010B multifunction on Trade Me for a good price. The meter is in as new condition and has hardly been used. This was a far better option than some of the cheaper insulation resistance testers available on-line that all seem to be made in the same factory in China but with different brand names. All sorted now.