please help with this question

[paulthemagpie]

hi guys,

i need some help with an assignment and id appreciate it so so much if anyone can help me.

A circut which is supplied from DB 3 is protected by a 16A type C circut breaker and has a measured earth fault loop impednace of 1.35 ohms. Please determine (with reasons were applicable)

a. if this value of earth fault loop impendance is acceptable
b. at least one method of achieving disconnection should the device provide unacceptable in (a.) above with out re-wiring the circut.
c. at least one method of confirming the integrity of the live conductors of that circut under short circut conditions.

thanks alot guys for any help

 

[plugsandsparks]

simply reading the table the max for 16A is 1.44 so the 1.35 is acceptable

Close but not there yet. The question states this was MEASURED, the above number is a theoretical figure see my previous post

 

[paulthemagpie]

and then is c the use of RCMs

 

[paulthemagpie]

No, not really. The current you are looking for is the one which makes sure the breaker trips in 0.4s, hint its much much higher than 16A, i.e. the rating of the breakers and if you think about it, it would be a pretty annoying breaker if it did trip in 0.4S at 16A.

Also there is a rule of thumb used here cos its hard to measure when its all running at max temp and current so you measure at ambient, often with low or no load.

hi,

thanks for your help, but you lost me now!

according to the table 41.3 the max for a 16A is 1.44 ohms so i thought the measured reading of 1.35ohms would mean its withing the max allowed?

 

[Richard Burns]

As plugandsparks says for the value in the regs books this is at 70oC and you need to correct as Malcolm has said in post 3 a for the temperature at which the EFLI has been measured.

for B you are pretty much there but why is this an alternative.

for C you are, I think, on the wrong track, this is looking for a check you can make.

 

[plugsandsparks]

hi,

thanks for your help, but you lost me now!

according to the table 41.3 the max for a 16A is 1.44 ohms so i thought the measured reading of 1.35ohms would mean its withing the max allowed?

Read the note below that table, it refers to an Appendix where all the answers are waiting for you.

 

[paulthemagpie]

for question a.

so the measurement so far is the measured figure then we need to take into account the increase in resitance with increase of temp

 

[Richard Burns]

Yes the measured earth fault loop impedance of 1.35 ohms is measured at, presumably, 20oC.
The maximum EFLI for a 16A type C BS60898 breaker is given in the regs as 1.43 ohm, as measured at 70oC.
Apply the correction factor to the tabulated value to determine the equivalent value if it was measured at 20oC.
Then compare this value with your measured value to see if it complies.

 

[paulthemagpie]

oh so the figures in the book are for when at 70oC?

i cant find were it states that!

 

[brman]

Do you have the on site guide? If so, have a look at page 114 - it explains the difference between measure and design Zs more clearly that the BGB I think. Be warned if comparing the two as the tables in the OSG already have the correction factor applied.

 

[Richard Burns]

oh so the figures in the book are for when at 70oC?

i cant find were it states that!

See the note at the bottom of the table this states that the conductors are at their normal operating temperature and tells you to look at appdx. 14.

 

[plugsandsparks]

oh so the figures in the book are for when at 70oC?

i cant find were it states that!

I dont think it does explicitly, it does say increase in temp due to load current and typically cables are good for 70degress, the formula given in App14 is only trying to give a nice fudge factor and since the theoretical figures are derived from U0/Ia using the Ia's in App3 you can see from that where the 1.43 comes from and hence where the measured (rule of thumb) comes from

 

[brman]

oh so the figures in the book are for when at 70oC?

i cant find were it states that!

I have to say I think the way it is explained in the regs is a bit confusing.
The tables on the BGB do not quote a temperature as they are the figures to be used at operating temperature which will depend on the installation. If you think about it, all the Max Zs is is the resistance required to get enough current to trip the MCB in the required time. So if you need 160A to trip in 0.1 to 5 seconds (your 16A type C) then the resistance is 230V / 160 = 1.44 ohms.
Logically that Max Zs never changes - regardless of temperature you need a Zs less than that because you always need that current.

However the correction factor is used to change the Max Zs to the Max measured Zs. As the temperature when measuring will be lower than when operating you have a smaller Max measured Zs than the tables.

 

[brman]

btw. for b) your answer of and RCD is ok but you could also consider how you could reduce the Zs of the circuit without replacing the cable. Think about earthing.....

For c) I'd be curious to know what people think as I am not clear whether it is asking how to measure the continuity of the conductors or whether the conductors will be capable of taking a fault current or how to monitor the condition of the conductors in service.

 

[malcolmsanford]

btw. for b) your answer of and RCD is ok but you could also consider how you could reduce the Zs of the circuit without replacing the cable. Think about earthing.....

For c) I'd be curious to know what people think as I am not clear whether it is asking how to measure the continuity of the conductors or whether the conductors will be capable of taking a fault current or how to monitor the condition of the conductors in service.

The OP did state for b) without rewiring, so the only way you could conform with disconnection times in that case would be by RCD.

Agree with you about c) it is a strange question.

 

[Richard Burns]

I think for C they are just looking for an insulation resistance test, this would confirm that the cables have suitable integrity, though why it mentions a short circuit I do not know.