The measured value of loop Impedance for a circuit is 2.4 Ohms

[Rather be Fishing]

Hello trainee forum thank you very much for letting me in here and as a consequence hopefully helping me to develop my skills and knowledge.

I'm currently focusing on the 2391 and have enrolled myself on a course with no previous Inspection and testing experience or Knowledge so I do feel kind of up against it but ill give it my best shot and see how many re sits it takes me.

First of all I have to pass a multiple choice that requires you to get 75% and I'm currently doing some homework for it but my notes are leaving me unable to answer this question.

If anyone could help me it would be much appreciated.

The measured value of loop impendence for a circuit is 2.4 ohms. If the temperature at the time of the test was 20 degrees C and the cable is 70 degrees C (factor 1.2) what is the corrected value. Ze =0 .4ohms

Here are the possible answers they give you to choose from

A)2.88ohms
B)2.8 ohms
C)2.4 ohms
D)2.0 ohms


Ill show you what I have written down

Zs = 2.4
Ze = 0.4
R1 + R2
R1 + R2 x 1.2 = 2.4ohms
=0.4ohms

= 2.8ohms


Does anyone know if my math's is right as it doesn't feel right to me as when I apply it to similar questions the answer is always wrong.

 

[westward10]

Your answer is correct but your maths isn't clear.
Zs - Ze = 2.0
2.0 × 1.2 = 2.4
2.4 +Ze = 2.8

 

[Rather be Fishing]

Thank you @westward10 much appreciated mate.

I thought I would apply it to another question that I got wrong

The measured value of loop impedance for a circuit is 0.83ohm. If the temperature at the time of the test was 20C and the cable is 70C (factor 1.2) what is the corrected value. Ze = 0.4ohm:

a) 0.43 ohm
b) 0.86 ohm
c) 0.996 ohm
d) 0.916 ohm

Zs-Ze
= 0.83 - 0.4
= 0.43

0.43 x 1.2 = 0.516

0.516 + 0.4

= 0.916 ohms

which is what they said the answer was


so thank you very much you've just taught me how to do something

 

[westward10]

No worries if you are stuck on anything post it up with your workings and answer.

 

[Rather be Fishing]

You are performing the initial verification on an installation but are concerned with the length of the lighting circuit conductors. Because of your concerns you have decided to verify the circuits voltage drop. The circuit is wired in 1.5mm2 conductors and runs for a circuit length of 48m.

You have measured the cold R1+Rn value for the circuit and found it to be 1.18ohms.

The designer has advised the calculated voltage drop for the circuit is 4.3V

If the design current is 3.04A, does the circuit comply with the design requirements?

Here is what I seem to have written down

R1 Rn = 1.18ohms x 1.04 ohms = 1.2272 ohms

voltage drop = 3.04 x 1.2272 = 3.730688

yes it complys

Wrong

R1 Rn = 0.63 ohms x 1.2 = 0.756
16 x 0.756 = 12.096V

5% = 11.5 (230/100=2.3V)

2.3 x 5% = 11.5

No it does not comply

I'm not sure if I have the right answer there or not its just what I wrote down but now I'm going back over it even if I did have the right answer I don't really follow it.








3.04it complys

 

[westward10]

Where have you got 1.04 and 0.63 ohm from.
You need to verify your measured R1 + Rn is correct in relation to the design length of 48m.

 

[Lister1987]

Where have you got 1.04 and 0.63 ohm from.
You need to verify your measured R1 + Rn is correct in relation to the design length of 48m.

The 1.04 and 0.63 are the poster doubling the R1+Rn value, presumably thinkibg it was a single conductor reading (R1 or Rn)

 

[Rather be Fishing]

Where have you got 1.04 and 0.63 ohm from.
You need to verify your measured R1 + Rn is correct in relation to the design length of 48m.

I have no Idea I was wondering that to

 

[westward10]

I could be wrong here but your known design facts are length, volt drop and design current. R1 + Rn is your measured figure so I believe that once you have established this is correct you can double check it all meets the volt drop criteria (3%).

 

[Rather be Fishing]

Where have you got 1.04 and 0.63 ohm from.
You need to verify your measured R1 + Rn is correct in relation to the design length of 48m.

I have no Idea I think I might have just been having ago at it having been given the question in a lesson

Looking at my notes it gives me a couple of worked out example's. One using method1 and the other using method2

I seem to remember being told method 1 is the easier of the two methods so I'm going to go with that one

After linking the line and neutral in the distribution board, a cold continuity reading of 0.78ohms was obtained from a circuit with a design current (Ib)of 14A, wired using standard twin and earth cable.

Using table B3 the following calculation is performed

R1=Rn = 0.78 ohms x 1.20 = 0.936 ohms

using ohms law the voltage drop is established as

Voltage drop = 14 x 0.96 = 13.10v

 

[Rather be Fishing]

You are performing the initial verification on an installation but are concerned with the length of the lighting circuit conductors. Because of your concerns you have decided to verify the circuits voltage drop. The circuit is wired in 1.5mm2 conductors and runs for a circuit length of 48m.

You have measured the cold R1+Rn value for the circuit and found it to be 1.18ohms.

The designer has advised the calculated voltage drop for the circuit is 4.3V

If the design current is 3.04A, does the circuit comply with the design requirements?

So

R1 Rn = 1.18ohms x 1.04 (because they are singles so not incorporated in a cable) = 1.2272

voltage drop = 3.04 x 1.2272 = 3.730688V

The designer said volt drop allowed is 4.3V

so yes it complies as its 3.730688V

 

[westward10]

Or use vd = (R1 +Rn ) × Ib × multiplier.

 

[Rather be Fishing]

Or use vd = (R1 +Rn ) × Ib × multiplier.

Thank you @westward10 going to jot that down

 

[Rather be Fishing]

Im trying to use my regs book to find the answer to this question but I'm struggling to hone in on it. Would anyone be able to point me in the right direction so I can find the answer for myself?

Periodic Inspection and testing of a mobile catering unit is to be carried out as a requirement of the clients insurer?

The supply is from a 3KVA 230V single - phase portable generator and is connected as a TN-S system with fault protection provided by BS EN 61008 30mA RCD

What is the maximum test current applied to the RCD to confirm fault protection is provided.

a)15 mA
b)30 mA
c)60 mA
d)300 mA

I would guess d but I would prefer to see it in the reg's book instead of going with a number that's in my head for some reason. If 150 mA would have been an option id have been in a dilemma if I was to guess at it.

 

[westward10]

You need to understand with regard to a 30ma rcd the difference between fault and additional protection. Knowing this will give you the answer.

 

[Rather be Fishing]

Is it fault for fire protection and basic protection, protection against electric shock

 

[westward10]

Fault protection in this case is where such protection (ADS) may not be achieved by a protective device alone.

 

[Rather be Fishing]

Continuity of a ring final circuit has been tested as part of an initial verification of a new primary school. All the sockets outlets are connected directly to the ring.

The circuit loop length is 58m long wired in 4mm2 live and 1.5mm cod flat profile cable.

The circuit is protected by a 32 A BS EN 60898 type b circuit breaker

The installation has a measured ze of 0.29 ohms

What is the expected as for this circuit?

A. 0 24 ohms
B.0 28 ohms
C.0.53 ohms
D. 0.58 ohms

I've done some calculations but I'm not getting any of those answers which means im doing it wrong.

This is what I have done

Zs = ze + R1 + R2

16.71 x 58 /1000 = 0.96 ohms


O.29ohms + 0.96 ohms

Zs = 1.25 ohms

Obviously that is not right

 

[westward10]

You have done it as a radial circuit not a ring final.

 

[Rather be Fishing]

You have done it as a radial circuit not a ring final.

You have done it as a radial circuit not a ring final.

Other then divide it by 2 I've no idea what I need to do

 

[Lister1987]

Have a look through, compare against yours and see where you may have gone wrong. Come back and tell us once you've figured it out (where you went wrong, not the answer)

 

[westward10]

Remember when the ring final is connected your resistance is halved due to double csa but the distance is effectively halved.

 

[Rather be Fishing]

Nice one @Lister1987

Pen and pad is the way foward for sure

I have to get to bed really as im up early and Ive had a long day with college thrown back into the mix but I'll be cracking straight back on with this tommorow.

Cheers for going to the trouble of writing that out

And also to @westward10 for much the same reasons

 

[Lister1987]

Nice one @Lister1987

Pen and pad is the way foward for sure

I have to get to bed really as im up early and Ive had a long day with college thrown back into the mix but I'll be cracking straight back on with this tommorow.

Cheers for going to the trouble of writing that out

And also to @westward10 for much the same reasons

Just remember to be methodical, helps to give sources too; Table G1 for R1+R2 values, Table G2 for the multiplier etc, it helps it get enground in the mind.

 

[Rather be Fishing]

I'll be cracking straight on with this after work and something to eat.


My plan was to spend all day sat and sun on revision as well as the remainder of this week but I've just been put on call for my new Job starting Friday. You couldn't write it. I bet it's one of them weekends.

 

[telectrix]

Other then divide it by 2 I've no idea what I need to do

divide by 4: (R1+R2)= (r1+ r2)/4 for a ring.

 

[westward10]

divide by 4: (R1+R2)= (r1+ r2)/4 for a ring.

He is supposed to be trying to work it out?

 

[Rather be Fishing]

Thanks for all the help guys can't quite believe it.

 

[Rather be Fishing]

Here we go

 

[Rather be Fishing]

Here's another one

 

[Rather be Fishing]

Because it said caravans I've gone to page 265 of the regs and started reading.

Reg 708.415.1 says

(Please see pic)

So does that mean the answer is a) 30ma

Or am I missing something to do with them dropping in TT system in the question along with some test results.

 

[Lister1987]

Because it said caravans I've gone to page 265 of the regs and started reading.

Reg 708.415.1 says

(Please see pic)

So does that mean the answer is a) 30ma

Or am I missing something to do with them dropping in TT system in the question along with some test results.

Working on the basis that this reads as being the main source of supply for the park, (the caravan park bit is a red herring I believe) It is asking about FAULT protection, not Additional (30ma). I'd select 100ma (maximum EFLI 500 ohms, higher than 167ohms for 300ma.

Add the 3 together divide by 3 gives you an EFLI of 407, which is within limits of the 500mA bit outside those of 300mA and 30mA .

Please correct me if I'm wrong ?

 

[Rather be Fishing]

Periodic Inspection and testing of a mobile catering unit is to be carried out as a requirement of the clients insurer?

The supply is from a 3KVA 230V single - phase portable generator and is connected as a TN-S system with fault protection provided by BS EN 61008 30mA RCD

What is the maximum test current applied to the RCD to confirm fault protection is provided.

a)15 mA
b)30 mA
c)60 mA
d)300 mA

You need to understand with regard to a 30ma rcd the difference between fault and additional protection. Knowing this will give you the answer.

I don't know if I've gone to the right place but I've started of at page 55

when I get to reg 411.1 I can see 30mA at the bottom of it and I was told the answer was b) 30mA but I thought I might as well carry on and confirm the difference between fault and additional protection.

In my head

Fault protection is to try to save human and livestock life if there is a electrical fault and they happen to come in to contact with it.

Reading the regs, regulations 411.3.1 and 411.3.2 seem to apply. Protective earthing and Automatic disconnection incase of fault (ADS).

To me this is saying that fault protection is provided by your supplier, all of those typically green and yellow cables (metal conduit or SWA protective strands perhaps). And then the MCB which is already designed and built for you.

Addition protection is that that is Provided by a RCD again designed and built for you and that is required for socket outlets not exceeding 32A and mobile equipment with a rated current not exceeding 32A for use outdoors. reg 411.3.4 adds lights in a domestic premises must also have RCD protection.

Other than seeing 30mA in regulation 411.1 which was on the answers list and not seeing any of the others all the way through to chapter 42 b) 30mA would be the one id have gone for if I had been able to find it in the exam because I couldn't in the mock and went with eni meni d) 300mA. I don't know if someone can help me reassign this 300 figure (maybe ms) to something else in my brain but I'm sure it applies to something because it in there

In my head I've got 1/2x 1x and 5x which is the RCD tests. 30 x 5 is 150mA which isn't on the answers list luckily is higher then 1x 30mA

Is that saying that when you 1x test a RCD you are confirming fault protection.

That just makes me wonder what protection 5x is confirming

 

[Rather be Fishing]

Working on the basis that this reads as being the main source of supply for the park, (the caravan park bit is a red herring I believe) It is asking about FAULT protection, not Additional (30ma). I'd select 100ma (maximum EFLI 500 ohms, higher than 167ohms for 300ma.

Add the 3 together divide by 3 gives you an EFLI of 407, which is within limits of the 500mA bit outside those of 300mA and 30mA .

Please correct me if I'm wrong ?

I'm a bit confused but I like the math's there

Is there a page in the regs that lists the maximum rating of RCD that can be used in fault protection for any given system TT, TNS, TNCS.

I'm just think out loud but would you typically only find those larger rated mA rcd's in a TT system or am I not quite understanding something as they could be found in other places

 

[Rather be Fishing]

Basically I need to store max rating of RCD that can be used for fault protection on a TT system is 300mA and for a TNS is 30mA in my head. ]