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Topic Title: Measuring Volt Drop
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Created On: 03 July 2013 10:08 PM
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 03 July 2013 10:08 PM
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MrOther

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If I want to measure for volt drop at an appliance - as I don't know the cable lengths (so calcs is not possible) am I right in thinking the results I'm getting from my Fluke T5 1000 (an averaging meter) when testing for voltage don't represent volt drop incorporated in their result?
 03 July 2013 10:27 PM
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daveparry1

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Just measure at the end of the circuit with no load on it them measure again with the load applied, the difference in readings is the volt drop,

Dave.
 03 July 2013 10:46 PM
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MrOther

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Dave, I installed some LEDs at Christmas on a Dimmer.

One has already gone.

Are LEDs more tempremental when dimming? Do the electronics really like it? Even if they are made to do it?

Just used TLC VD Calc as a quick guide and I'm looking at:

500w at 230v, cable length of 100m with a VD 2.7% - well there isn't 100m in that install. And even if their were I'd be very surprised if 500w is ever present on the circuit - unless every light is on fall blast, Discriminations says otherwise.

So do I conclude VD has nil to do with it and it's just a bad LED?
 03 July 2013 11:00 PM
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daveparry1

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Yes I think so MrOther, I don't think you have a volt-drop problem there,

Dave.
 03 July 2013 11:36 PM
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M.Joshi

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Originally posted by: MrOther

Are LEDs more tempremental when dimming? Do the electronics really like it? Even if they are made to do it?


You have to ensure firstly that they are dimmable and secondly, the correct type of dimmer is used otherwise, they can fail.

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M.I.E.T - Forfeited this due to The I.E.T's ridiculous membership rules!
 04 July 2013 07:01 AM
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zeeper

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as I don't know the cable lengths (so calcs is not possible)


Simply measure the resistance of the line and neutral conductors as shown below

Volt Drop = Ib x (R1+RN) x 1.2
 04 July 2013 09:16 AM
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broadgage

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To measure voltage drop you simply measure the voltage on load and off load, and calculate the difference.
Off load=244 volts, on load=239 volts, voltage drop is 5 volts.

The tests should be repeated several times since other wise misleading results may be obtained if the supply from the DNO varies between taking the readings.

In the example above, consider what would happen if the DNO voltage increased by 4 volts between recording the off load and the on load voltage. That would result in the voltage drop appearing to be 1 volt not 5 volts.

A better approach is to take the readings at the same time, at the load and at the origin of the circuit, but that needs two instruments, and usually two people.
If useing two instruments, swap them over and repeat the tests since otherwise small instrument errors can become significant.
If one instrument reads one volt high, and the other reads one volt low, then the voltage drop in the above example could appear to be either 3 volts, or 7 volts. An average of the two would give the correct figure of 5 volts.

Remember also that the DNO voltage will normally drop when the consumer uses more load, consider what exactly do you want to measure ?
The voltage drop ONLY in the subcircuit ?
Or the TOTAL voltage drop including that in any submains, in the service cable, and in the DNO network.
 04 July 2013 09:37 AM
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daveparry1

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Hardly any need to measure at the supply end as the vd on the cable when o/c (no load) would be negligible i'd say?

Dave.
 04 July 2013 09:56 AM
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broadgage

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Originally posted by: daveparry1

Hardly any need to measure at the supply end as the vd on the cable when o/c (no load) would be negligible i'd say?

Dave.


Voltage drop in the unloaded subcircuit cable would indeed be virtualy nothing.
One might however wish to measure the voltage at the origin of the circuit at the same time as the voltage at the load, in order to eliminate errors caused by the voltage from the DNO altering.

Also, a measurement at the origin of the subcircuit will be required if one wishes to know the voltage drop in ONLY the subcircuit, rather than the total drop including the service cable.
This can be significant, with a short subcircuit and a long service, voltage drop in service cable and in the DNO network can easily exceed the drop in the subcircuit.
 04 July 2013 01:25 PM
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zeeper

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To measure voltage drop you simply measure the voltage on load and off load, and calculate the difference.
The tests should be repeated several times since other wise misleading results may be obtained if the supply from the DNO varies between taking the readings.




just to point out to the op, that GN3 recommends not to use the method above.
 04 July 2013 06:56 PM
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geoffsd

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Is the figure of 1.2 for a specific reason or is that just what it happens to be?

Thinking resistance change 20 to 70 degrees.
 06 July 2013 08:50 AM
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zeeper

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Yes a correction for conductor temperature
Statistics

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