All about resistors...

[timhoward]

I've managed to mainly fix a Megger 1502 MFT that was so far out of calibration it got laughed out of a tester day.
I have high current loop. rcd test currents, and low resistance within spec now. (I can share more info about this process separately if anyone's interested, it's been an interesting learning curve).

IR results are just about within +-2% +-2 digits. To get them more accurate I need a 5M resistor with lower than 5% tolerance. I struggled to find anything cheap and these results were obtained using a 3M and 2M in series, they both have 5% tolerance and overall it's not quite good enough.
So the first easier question is any recommendations for better resistor accuracy, hopefully without spending a small fortune. I'm wondering if adding a trim pot is a good idea.

The 2nd question is more complex and regards the low-trip loop calibration which is the last real hurdle and may prove to be the brick wall I hit.
Part of the test passes 25A current between Live and Neutral as a basis for further maths.
Part of the calibration involves adding 10 ohms to the Neutral path and storing the result, and adding 10 ohms to the earth path and storing the result. No problem so far.
But then adding 10 ohm load to the Live path....On the face of it this would seem to need a resistor rated at 5.29kw ?! Or am I getting something basic very wrong? Obviously this is lethal enough anyway and I will admit defeat and draw stumps if I can't be sure what is required here.
Thanks for any thoughts.

Excerpt from the service manual:

 

[mainline]

I've managed to mainly fix a Megger 1502 MFT that was so far out of calibration it got laughed out of a tester day.
I have high current loop. rcd test currents, and low resistance within spec now. (I can share more info about this process separately if anyone's interested, it's been an interesting learning curve).

IR results are just about within +-2% +-2 digits. To get them more accurate I need a 5M resistor with lower than 5% tolerance. I struggled to find anything cheap and these results were obtained using a 3M and 2M in series, they both have 5% tolerance and overall it's not quite good enough.
So the first easier question is any recommendations for better resistor accuracy, hopefully without spending a small fortune. I'm wondering if adding a trim pot is a good idea.

5M Ohm 1% 1W 1.5Kv High voltage resistor TESLA Qty. 5 | eBay

High stability 100 ppm. Part number RNX-3/8.

www.ebay.co.uk

The 2nd question is more complex and regards the low-trip loop calibration which is the last real hurdle and may prove to be the brick wall I hit.
Part of the test passes 25A current between Live and Neutral as a basis for further maths.
Part of the calibration involves adding 10 ohms to the Neutral path and storing the result, and adding 10 ohms to the earth path and storing the result. No problem so far.
But then adding 10 ohm load to the Live path....On the face of it this would seem to need a resistor rated at 5.29kw ?! Or am I getting something basic very wrong? Obviously this is lethal enough anyway and I will admit defeat and draw stumps if I can't be sure what is required here.
Thanks for any thoughts.

I would have thought that the measurement is done very fast.

 

[timhoward]

would have thought that the measurement is done very fast.

That is one of the many competing thoughts I’ve had.
At the moment the highest rated resistor I have is 300w. One thing is for sure - if I eventually go for it I’ll film it!

 

[mainline]

That is one of the many competing thoughts I’ve had.
At the moment the highest rated resistor I have is 300w. One thing is for sure - if I eventually go for it I’ll film it!

Megger 249000 CALIBRATION RESISTOR 10 Ohm | OMNICONTROLS

CALIBRATION RESISTOR 10 Ohm, current rating 0.001A

www.omnicontrols.com

Look at the current rating

 

[timhoward]

Megger 249000 CALIBRATION RESISTOR 10 Ohm | OMNICONTROLS

CALIBRATION RESISTOR 10 Ohm, current rating 0.001A

www.omnicontrols.com

Look at the current rating

Interesting! Thanks.

 

[mainline]

Interesting! Thanks.

It's meant for ohmmeter, so probably no good for what you want, but it looks massive compared to the current rating.

 

[timhoward]

I would have thought that the measurement is done very fast.

Well, you were spot on! Nothing exciting happened at all! The resistor wasn't even warm afterwards.
Thanks also for the link to the 5M resistor - now ordered.

I'm pleased to say that I've now got the No Trip loop test working as well as it ever does:

and with 50 ohms added using the check box, fairly close:

and finally the High Current test is exactly matching the calibrated Metrel:


I believe it's now close enough to not instantly be rejected at a tester day.

 

[timhoward]

I've now moved onto the next challenge - a long-abandoned 1730 that gives very low loop readings.
There are several easy ways to get a 1500 series into calibration mode. None of them work on a 17xx.
Last night I finally found a way using the service port and a PC connection. This was a cause of great celebration.
I'm starting to think a "Hacking Meggers" thread in a less public area of the forum might be of interest to the small select number of crazy people like me on here who can't bear to not know how something works...

 

[Moley]

I'm interested and I don't have a Megger MFT. I'm up for a hacking thread, not just for Meggers but all sorts of equipment.

 

[xmetal]

I would be interested too. I think the last thing I hacked was to use a hex editor to remove the reprogramming lock on a two-way radio.