I have two questions.

[Goody]

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 ->

 

[Deleted member 26818]

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

 

[Sparky Ninja]

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

 

[Sparky Ninja]

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

 

[Amp David]

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

 

[IQ Electrical]

I very much doubt if your shaver socket will be SELV

 

[Des 56]

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

 

[JUD]

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

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).

 

[Goody]

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

 

[JUD]

The R1+R2 = 0.57 ohms. Thanks

What size cable is it?

 

[spark 68]

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 ?

 

[JUD]

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 ?

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.

 

[spark 68]

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:

 

[JUD]

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.

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

 

[spark 68]

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

 

[IQ Electrical]

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

If you're asking for 2391-10 then you need to look at what values you are given:

1 The resistance per Metre of the cable OR

2 The mV/A/M figure from BS7671:2008

With option 1, you need to double the figure to allow for the neutral conductor, with 2 the figure is already included in the mV/A/M figure.

 

[spark 68]

Thanks IQ,

I have a big thick book of calcs somewhere, but it is fairly heavyweight mathematics.

I wanted to Know this for both the 2391-10 and to understand it for if I have to design for this.

As always there is more than one method, and I don't want to mix them up

 

[JUD]

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

No. The mV/A/m take into account the neutral.

They're worked out using the resistance of both line and neutral.

(R"1 + R"n) x Ct

R"1 = resistance per meter of line conductor
R"n = resistance per meter of neutral conductor
Ct = rating factor for operating temperature of conductor

Example

2.5mm² cable

R"1 = R"n = 7.41mΩ/meter at 20°C (Table 9A OSG)
Ct = 1.2 (corrects resistance for 70°C) (Table 9C OSG)

(7.41 + 7.41) x 1.2 = 17.78 (rounded up to 18)

mV/A/m for 2.5mm² is 18 (Table 6D2 OSG)

 

[spark 68]

Thanks Jud,

That clarifies things, I was just working it out another way, (possibly wrong) as I was not sure if the neutral return was taken into account.

The OSG (6D2) is the easier method.

 

[IQ Electrical]

Thanks IQ,

I have a big thick book of calcs somewhere, but it is fairly heavyweight mathematics.

I wanted to Know this for both the 2391-10 and to understand it for if I have to design for this.

As always there is more than one method, and I don't want to mix them up

It caught a few people out on one of last years 2391-10 papers, just giving conductor resistance values.

 

[JUD]

Thanks Jud,

That clarifies things, I was just working it out another way, (possibly wrong) as I was not sure if the neutral return was taken into account.

The OSG (6D2) is the easier method.

Yes. Still use 6D2.

My post above is just to let you know how the values in 6D2 are worked out.

 

[MarkieSparkie]

It's worth pointing out to apprentice and new sparks that GN3 does not recognise direct measurement of voltage drop with a voltmeter. It assumes voltage drop will be determined by calculation, either from the design data if available or from the measured circuit impedance. The calculated value is then compared with the limits in BS7671, Appendix 12. The values in Appendix 12 may be exceed in periods of motor starting or high inrush current, this may acceptable if it is within the relevant product standards or the manufacturers data.

 

[malcolmsanford]

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

I think the Mathematics have side tracked us somewhat here, so back to you Goody ................why.

What record do you need to find this out for, where are you recording this for your CU change.