I have two questions.

Goody

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• Jul 24, 2011

• #1

1. I am testing “Voltage Drop” at the mini DB where the SWA cable from the main DB terminates into and I am not seeing much of a difference between Voltage readings at the mini & main DB’s. The cable run is around 40 ft. Could I use the test lead with 3 pin plug on my Fluke 1652 on live sockets at the garage and at the house to check the voltage (with some load on the circuits)? What is the best way to get an accurate reading of voltage drop? Also, because this lead has no fuse in it (not sure if it does), would I damage the meter?
2. I have a mirror light with integrated 2 pin shaver socket (SELV). What tests I do on it and how do I go about doing them? Many thanks!

3 pin plug on my Fluke 1652 ->

 

D

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• Jul 24, 2011

• #2

What's the design current for the circuit, and what's your R1 + R2?

 

Sparky Ninja

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• Jul 24, 2011

• #3

Calculate the voltage drop with either the circuit design data or a measurement of the circuits impedance.

 

Sparky Ninja

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• Jul 24, 2011

• #4

If you click the link in my sig and download chapter 7 there are some slides on insulation tests on selv systems.

 

Amp David

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• Jul 24, 2011

• #5

For question number 1, http://www.electriciansforums.net/i...al-installations/38061-testing-volt-drop.html

 

IQ Electrical

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• Jul 24, 2011

• #6

I very much doubt if your shaver socket will be SELV

 

D

Des 56

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• Jul 24, 2011

• #7

If you test the voltage at the mini board with no load connected you will get very little if any volt drop reading
When a load is applied thats when it would become evident

 

J

JUD

• Jul 24, 2011

• #8

Des 56 said:

If you test the voltage at the mini board with no load connected you will get very little if any volt drop reading
When a load is applied thats when it would become evident

Click to expand...

And the load applied would need to be your full design current in order to get an accurate reading.

Just calculate it as widdler said (which you should have already done when designing the circuit).

 

OP

Goody

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• Jul 24, 2011

• #9

Thanks, folks. I have not designed this circuit but it was already in place and I have done a CU change as part of assessment; I needed to find this out for the record. The R1+R2 = 0.57 ohms. Thanks

 

J

JUD

• Jul 24, 2011

• #10

Goody said:

The R1+R2 = 0.57 ohms. Thanks

Click to expand...

What size cable is it?

 

spark 68

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• Jul 24, 2011

• #11

Hi,
If you are calulating voltage drop, should you not be measuring R1+Rn, and then looking at the design current and then the tables in the BRB and calculating (mV/A/M)/1000 ?

 

Last edited: Jul 25, 2011

J

JUD

• Jul 24, 2011

• #12

spark 68 said:

Hi,
If you are calulating voltage drop, should you not be measuring R1+Rn, and then looking at the design current and then the tables in the BRB and calculating mV/A/M ?

Click to expand...

You're only using continuity results to determine the length of the cable run. This can be done using R1 + R2 results and dividing by the resistance per meter from Table 9A OSG for whatever size the line and cpc are.

 

Last edited by a moderator: Jul 24, 2011

spark 68

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• Jul 24, 2011

• #13

Hi Jud,
yes I get that bit, I suppose you could just double your R1, I was thinking more about calculating the volts drop on T+E, where your CPC can be smaller, or if the armour of an armoured cable was used as a cpc.

Trying to learn for my 2391 :28:

 

Last edited: Jul 24, 2011

J

JUD

• Jul 24, 2011

• #14

spark 68 said:

Hi Jud,
yes I get that bit, I suppose you could just double your R1, I was thinking more about calculating the volts drop on T+E, where your CPC can be smaller, or if the armour of an armoured cable was used as a cpc.

Click to expand...

Table 9A gives you the resistance per meter for cables with smaller cpcs -- eg. 2.5mm² + 1.5mm² = 19.51mΩ/meter (2.5mm² = 7.41mΩ/meter & 1.5mm² = 12.10mΩ/meter -- 12.10 + 7.41 = 19.51mΩ/meter)

You would the divide your R1 + R2 result by 19.51 ÷ 1000 (0.01951) and that would give you the length of the cable

 

Last edited by a moderator: Jul 24, 2011

spark 68

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• Jul 25, 2011

• #15

What I was asking (sorry to jump on to someone elses thread), was when calculating volts drop, is do you need to double the length to take into account the return via the neutral in your (mV/A/M)/1000 calculation ?

I cannot find where the calcs are for this at the minute, I cannot see it in the BRB or GN3, it may be in unites red book