2391 help needed

[searley]

can anyone help with these questions i have on a paper ?

1,the following readings were obtained during the inital test on a ring circuit

L1to L2= 0.8ohms ,n1 to n2= 0.8ohms, cpc to cpc- 0.8ohms

a, what readings would you expect after cross connection

1, between L & n at each socket

2, between L & cpc at each socket

b, what does L to cpc reading at each socket outlet represent

2 the following results were obtained on a ring circuit continuity test

test 1 L1 to L2 = 0.3ohms n1 to n2 =0.3ohms cpc1 to cpc2 =0.5ohms

test 2 the reading at each socket was 0.15ohms

test 3 the reading at each socket outlet was 0.2ohms

show by means of calculation weather the ring has been correctly wired or not,which readings should be recorded and the name of the form to be used for the purpose.

any help would be welcome.

 

[Robin Spark]

can anyone help with these questions i have on a paper ?1,the following readings were obtained during the inital test on a ring circuit

L1to L2= 0.8ohms ,n1 to n2= 0.8ohms, cpc to cpc- 0.8ohms

a, what readings would you expect after cross connection

1, between L & n at each socket (0.8+0.8)/4=0.4ohms

2, between L & cpc at each socket(0.8+0.8)/4= 0.4ohms

b, what does L to cpc reading at each socket outlet represent R1 + R2

2 the following results were obtained on a ring circuit continuity test

test 1 L1 to L2 = 0.3ohms n1 to n2 =0.3ohms cpc1 to cpc2 =0.5ohms

test 2 the reading at each socket was 0.15ohms

test 3 the reading at each socket outlet was 0.2ohms

show by means of calculation weather the ring has been correctly wired or not,which readings should be recorded and the name of the form to be used for the purpose.

using formulae (r1+rn)/4

then (0.3+0.3)/4= 0.15ohms, so test 2 gives correct result

Using formulae (r1 + r2)/4

then (0.3+0.5)/4=0.2ohms, so test 3 gives correct anticipated result

any help would be welcome.

Therefore the ring cct has been correctly wired and on the Schedule of Test Results sheets would be used recording R1+R2 which is 0.2 ohms

 

[searley]

thanks for the last answers got most of it write just needed someone who nose the biz to check them over.college lecture off for two weeks and no answer sheets to check with nightmare. cheers

could you take a look at this one for is ?

an insulation resistance test between live conductors on five individual circuits from a consumer unit produced the following results

CIRCUIT Mohms

lighting 250.0

ring circuit 25.0

water heater 2.0

cooker 1.0

radial circuit 1.14

what would be the insulation resistance reading taken at the supply terminals with all fuses in and all lamps etc removed ? state if the overall test results conforms to bs 7671

cheers.

 

[Robin Spark]

thanks for the last answers got most of it write just needed someone who nose the biz to check them over.college lecture off for two weeks and no answer sheets to check with nightmare. cheerscould you take a look at this one for is ?

an insulation resistance test between live conductors on five individual circuits from a consumer unit produced the following results

CIRCUIT Mohms

lighting 250.0

ring circuit 25.0

water heater 2.0

cooker 1.0

radial circuit 1.14

what would be the insulation resistance reading taken at the supply terminals with all fuses in and all lamps etc removed ? state if the overall test results conforms to bs 7671

cheers.

Hi Searley,

well looking at those test results before even doing any calculations then we know its going to fail the minimum IR value of 1megohm as all the ccts are in parallel and therefore once we have worked out the overall IR value as a whole then the common rule will dictate that this value will be less than the minimum individual value of the cooker cct 1Megohm.

Do you remember Resistors in parallel formula

1/Rt= 1/r1+1/r2+1/r3...

Therefore with your results do this

firstly find common denominator which I have used 28500 as all of your values can be divided into this.

so

1/Rt= (114+1140+14250+28500+250)/28500

1/Rt= 44004/28500

then transpose formula

Rt = 28500/44004 = 0.647MegOhms which indicates a fail for the IR test

I hope this all makes sense to you, but if not I can clarify or try to explain further.

 

[a1]

2 the following results were obtained on a ring circuit continuity test

test 1 L1 to L2 = 0.3ohms n1 to n2 =0.3ohms cpc1 to cpc2 =0.5ohms

test 2 the reading at each socket was 0.15ohms

test 3 the reading at each socket outlet was 0.2ohms

show by means of calculation weather the ring has been correctly wired or not,which readings should be recorded and the name of the form to be used for the purpose.

any help would be welcome.

TEST 1:

Don't forget that the reason for the cpc is 0.5ohms is that it has a smaller csa of that of the Line and Neutral conductors. Typically it should be times by 1.67. (0.3 x 1.67=0.501ohms

TEST 2:

r1 + rn / 4 = 0.3 + 0.3 / 4= 0.15ohms

TEST 3:

r1 + r2 /4 = 0.3 + 0.5 / 4 = 0.20ohms

 

[a1]

thanks for the last answers got most of it write just needed someone who nose the biz to check them over.college lecture off for two weeks and no answer sheets to check with nightmare. cheerscould you take a look at this one for is ?

an insulation resistance test between live conductors on five individual circuits from a consumer unit produced the following results

CIRCUIT Mohms

lighting 250.0

ring circuit 25.0

water heater 2.0

cooker 1.0

radial circuit 1.14

what would be the insulation resistance reading taken at the supply terminals with all fuses in and all lamps etc removed ? state if the overall test results conforms to bs 7671

cheers.

1/rt = 1/250 + 1/25 + 1/2 + 1/1 + 1/1.14 =

1/rt = 0.004 + 0.04 + 0.5 + 1 + 0.877 = 2.421Mohm

1/2.421 = 0.413Mohm = FAIL.

It is also worth remembering that when you have them connected in Parallel, the total will ALWAYS be less than the smallest individual reading - In this case 1Mohm.

 

[a1]

Actually - All that I have said above has been covered. Pays to read it all.

How come our IR overall values differ then?

 

[Robin Spark]

Actually - All that I have said above has been covered. Pays to read it all. How come our IR overall values differ then?

Because i made the mistake, erm excuse doing maths late at night isnt always the best idea.

When I divided the radial cct of 1.14 into comon denominator of 28500 I used the figure of 250 instead of 25000.

So going back and redoing calc

1/Rt= (114+1140+14250+28500+25000)/28500

1/Rt= 2.42119

Therefore Rt= 1/2.42119 = 0.413Meg ohms

Apologies for the confusion

 

[a1]

That's OK mate - I thought I had made an error somewhere - I double and triple checked and came up with the same answer.