proving units

[ban-all-sheds]

If, for example, you earthed the midpoint of the secondary, then the voltage at each end would be halved.

The voltage at each end with reference to what?

 

[BS3036]

If, for example, you earthed the midpoint of the secondary, then the voltage at each end would be halved.

The voltage at each end with reference to what?

OK - badly phased. The voltage at each end with reference to earth would be half what the voltage at one end would be if the other end were earthed.

This is in relation to the claim that using a 700 V proving device to test that a voltage tester which is testing for 230V ac made any sense.

I don't see how the fact that the ends of an isolating transformer may move in opposite directions if you earth the middle, can lead to the belief that a voltage tester sees 700V peak when connected across a 230V supply.

 

[ban-all-sheds]

OK - badly phased. The voltage at each end with reference to earth would be half what the voltage at one end would be if the other end were earthed.

Yup.

This is in relation to the claim that using a 700 V proving device to test that a voltage tester which is testing for 230V ac made any sense.

It doesn't.

I don't see how the fact that the ends of an isolating transformer may move in opposite directions if you earth the middle, can lead to the belief that a voltage tester sees 700V peak when connected across a 230V supply.

Fuzzy understanding can lead to erroneous beliefs...

 

[ericmark]

It you look at standard Martindale tester

it has 4 voltage ranges.
50-75
100-150
200-240
400-560
To prove the unit works you need 75, 150, 240, and 560 volt DC supply and you need to test both ways around to show all neons do work with either polarity.

To test on 400vac would test all neons worked so proving units must be over 230v to ensure the last neon it not faulty. But also of course the 50v neon although it lights with 400v may not work with a 50v supply. One side of neon should light to show DC volts also note the 50-75 refers to 50vac and 75vdc.

The kit

however from advert:-

Martindale’s PD690 is a portable, battery powered proving unit for the testing of contact type voltage detectors up to 690V. This unit is suitable for indicators with 230V and 415/440V AC or DC detection ranges, such as the Martindale VI13700G. The PD690 can also check Drummond and other filament lamps due to its output being sufficient to illuminate the bulb.

These units light All the neons and so show they all work. However it does not show any will work at the voltage being tested.

So in the main these units are a waste of time and space and only serve to satisfy the HSE and other safety people.

For 230vac I use the 150va 230 volt inverter with a 12v alarm battery as one I already have it. Two it also doubles for other uses. And three most important I then know it will light with 230vac not just with the 690v from propriety proving units.

If I was building a proving unit I would select 50v not 690v as I want to prove dead and at the end of the day it is the 50v neon not the 400-560v neon that is important.

This

unit to quote:-

When the probe tips are applied to the PU100 the proving unit automatically goes into auto-test mode and the output voltage will ramp up from 50V up to 690V and then gradually return back to 50V and finally off – The Voltage output is clearly indicated on the 5 LED indicators on the front of the unit.

This method of testing allows you to check the operation of your Voltage Detector at the various voltage levels and also ensures the battery life of the PU100 is preserved as long as possible.

Now that does prove the unit works and is even cheaper than martindale version.

I can't believe no one else has picked up on the problems with martindale version! However both units do say there output is enough to light bulb testers so it does seem you could use different make proving unit to the tester. However I don't like shocks even if the energy is low so I would use a tester using very little current. Plus it will then also work line to earth without tripping a RCD so even with auto resetting RCD's it will prove dead.

50 volt is still a bit of shock and I do have a tester that starts at 6v with LED not neon display but the battery less unit trips a RCD line to earth and the battery version needs a battery to work (12v) so is not as reliable so I used Voltmeter and Martindale tester and hope what one misses the other will catch.

But even with all these precautions I have still had a cable become live while working on it and so also use neon screwdrivers as terminal drivers. In an ideal world we would get out the plans and see exactly where it is feed from but to do that plans have to exist and be correct so any thing we can do to remove risk has to be done including working on dead cables as if they are live. Having said that on domestic premises one can normally switch off all power. In real terms the whole idea of proving dead is more for commercial use where all the power can't be turned off. I also would say we should never work alone but there has to be a limit and this is why we have the "Risk Assessment" and to just have the proving unit by ones side to satisfy the safety officer without doing a risk assessment is really burying ones head in the sand.