hey guys continuity testing

[roketmorris]

hey guys i am studying for my C&G level 3 and i have my practical inspection and testing next week and i cant find anywhere thae values that are aceptable when you read continuity on a lighting circuit for instances and they havent told us at college

i presume it is somethning to do with the resistivity of the cable and the area like
reswitivity devided by length times area or something anyone help me out??

 

[ezzzekiel]

continuity of circuit protective conductors?
if so then you are not looking for a set value as in ir testing but you would require a very low reading proving that continuity exists.

 

[roketmorris]

yes sorry protective conductors so there is no set rmaximum ?? you justhave to make a decision whether you think it is correct ore not

 

[CRAGS]

There is a way of getting a figure your looking for but you would need information like length CSA etc

 

[roketmorris]

hats wat i thought but you wont always no that if you are doing a periodic or inspection wud you so wat do you do then if you dont know length or anything???

 

[CRAGS]

Guess you would just have to make a estimated guess but its like you say you would not know for sure where or how long the cable is.

 

[roketmorris]

yeah thats good so i might think something is all right when it isn't that is quite stupid really ah well

 

[JUD]

Table 7.1 in the OSG gives maximum cable lengths for final circuits. Most of these values are worked out using Voltage Drop as the limiting factor but you can use these maximum lengths to work out the maximum R1 + R2 readings you should be expecting using the conductor resistance table on page 166 (OSG)

For example the maximum length for a 6A lighting circuit (load distributed) on a TN system wired in 1.5/1mm² cable on a Type B MCB is 90m (limited by Voltage Drop).

Using this you can now work out your maximum R1 + R2.

From the table on page 166 the R1 + R2 resistance for 1.5/1mm² is 30.2mΩ/meter (0.0302Ω/meter). However this value is at a conductor temperature of 20°C so you multiply it by 1.2 to correct it for a conductor temperature of 70°C (the maximum operating temperature of the cable).

This then gives you a vlaue of 36.24mΩ/meter (0.03624Ω/meter). So if you now multiply your 90m length by your R1 + R2 resistance / meter you will get your maximum R1 + R2.

(36.24/1000) * 90 = 3.26Ω

So as long as your reading at the end of the circuit is less than 3.26Ω then you're ok.

Hope that helps.

 

[pushrod]

If you are looking for acceptable values for R1 + R2 to ensure safe operation of MCBs then you can use appendix 2 of the OSG eg on page 103. table 2D gives you Zs values for different circuit breakers . If you had a lighting circuit with a type B breaker on a 6amp mcb the figure is 6.18 ohms for a suitable disconnection time. If you subtract the Ze (0.80 for tn-s or 0.35 for tn-c-s) then you have a maximum value for R1 + R2. If your value is 1 ohm below these values you know you are ok It is quick and interesting because it shows you that with very small circuit breakers you can have surprisingly high values that are still acceptable.


edit : forgot to add- i took my inspection and testing practical for the L3 C&G 2330 a couple of months back and this is what we were advised to check. There were won't be anything incorrect on the board (or shouldn't be) - that is for when you do the fault finding part! Good luck anyway.

 

[Jason Smith]

if we take continuity testing to its simplest, for instance on Bonding, we would roughly calculate its length, then from the tables as above, work out its expected resistance. If its about right, then we know the Bonding conductor is firstly continuous, and secondly not had a long run of say inapropriate sizes joined into it (has been known)- Thats itin its simplist form. you would disconnect at the met, then at the water pipe etc, test the length then tick the box !

 

[JUD]

If you are looking for acceptable values for R1 + R2 to ensure safe operation of MCBs then you can use appendix 2 of the OSG eg on page 103. table 2D gives you Zs values for different circuit breakers . If you had a lighting circuit with a type B breaker on a 6amp mcb the figure is 6.18 ohms for a suitable disconnection time. If you subtract the Ze (0.80 for tn-s or 0.35 for tn-c-s) then you have a maximum value for R1 + R2. If your value is 1 ohm below these values you know you are ok It is quick and interesting because it shows you that with very small circuit breakers you can have surprisingly high values that are still acceptable.


edit : forgot to add- i took my inspection and testing practical for the L3 C&G 2330 a couple of months back and this is what we were advised to check. There were won't be anything incorrect on the board (or shouldn't be) - that is for when you do the fault finding part! Good luck anyway.

You can do it this way but you must be aware of voltage drop.

So let say you have a 6A lighting circuit (Type B) wired in 1.5/1mm² T&E on a TN-S system. You take your max Ze of 0.8Ω from your max Zs of 6.18Ω. This leaves you with 5.38Ω.

Now using that figure to work out the length of the cable....5.38 / (29/1000) = 185m.

This is more than double the maximum length for voltage drop.

Just realised I've made an error in my last post:

Table 7.1 in the OSG gives maximum cable lengths for final circuits. Most of these values are worked out using Voltage Drop as the limiting factor but you can use these maximum lengths to work out the maximum R1 + R2 readings you should be expecting using the conductor resistance table on page 166 (OSG)

For example the maximum length for a 6A lighting circuit (load distributed) on a TN system wired in 1.5/1mm² cable on a Type B MCB is 90m (limited by Voltage Drop).

Using this you can now work out your maximum R1 + R2.

From the table on page 166 the R1 + R2 resistance for 1.5/1mm² is 30.2mΩ/meter (0.0302Ω/meter). However this value is at a conductor temperature of 20°C so you multiply it by 1.2 to correct it for a conductor temperature of 70°C (the maximum operating temperature of the cable).

This then gives you a vlaue of 36.24mΩ/meter (0.03624Ω/meter). So if you now multiply your 90m length by your R1 + R2 resistance / meter you will get your maximum R1 + R2.

(36.24/1000) * 90 = 3.26Ω

So as long as your reading at the end of the circuit is less than 3.26Ω then you're ok.

Hope that helps.

When using conductor resistances to determine cable length, there's no need to multiply by 1.2 as you will be testing at 20°C

Therefore the calculation would be....(30.2/1000) x 90 = 2.72Ω

 

[pushrod]

You can do it this way but you must be aware of voltage drop.

So let say you have a 6A lighting circuit (Type B) wired in 1.5/1mm² T&E on a TN-S system. You take your max Ze of 0.8Ω from your max Zs of 6.18Ω. This leaves you with 5.38Ω.

Now using that figure to work out the length of the cable....5.38 / (29/1000) = 185m.

This is more than double the maximum length for voltage drop which is why you should be looking for a reading under 3.26Ω as in my previous post.

As i was writing that i was thinking of you and voltage drop I suppose we were told it was ok because the circuits in the workshop are all so short - that and they didn't want to confuse us

Understand and appreciate what you are saying though


edit : you might think you are having a bad day JUD - i've just realised that the OP (roketmorris) wasn't even talking about R1+R2 values !!!

 

[albnegru]

Hello.i start a electrician cours domestic and industrial level 3 and i have a question on a test with out answer ( i'm not from this country sorry for my english)
if a continuity test gives a very low resistance reading it means that
a. there is a break in the conductor
b.the cable's termination are loose or corroded
c.the cable is too long
d.the cable is satisfactory

 

[Silly Sausage]

4th letter of the alphabet!

 

[albnegru]

that's mean the answer is d.?or i misunderstand ( i don't know too much english )

 

[Richard Burns]

Yes the answer is d.
A cable showing a low resistance reading on a continuity test demonstrates that the cable is continuous and so satifcatory.

 

[albnegru]

thx ! a socket wired as a spur has higher resistance than a socket outlet wired into the ring circuit? true or false

 

[telectrix]

assuming the same cable sizes, true because you have 2 conductors in parallel on a ring, therefore lower resistance.

 

[NDG Elecs]

Yes answer is D, Alb..

 

[Geordie Spark]

hats wat i thought but you wont always no that if you are doing a periodic or inspection wud you so wat do you do then if you dont know length or anything???

.......... and in English that is????????