Insulation Resistance Calculation

[Ben Murray]

Hi, I have an engineer who is completing his 2391-52 exam tonight and he has been told one of his mock questions was wrong, I have completed the working out myself and came up with the same answer as him, so either the paper is wrong or something has literally left my mind!!

"Determine the overall insulation resistance where the individual readings for four circuits are: 200M ohms, 100M ohms, 50M ohms & 20M ohms"

1/RT = 1/R1 +..... you get the picture.

1/200 = 0.005
1/100 = 0.01
1/50 = 0.02
1/20 = 0.05
Total = 0.085
1/0.085 = 11.76470588235294 (11.76 rounded)

The 4 multiple choice options are:
a) 1.68M ohms
b) 16.8M ohms
c) 16.8 ohms
d) 168 ohms

Please re-educate me and tell me where I am going wrong!

 

[telectrix]

well i get 11.7647, so youse correct.

 

[DPG]

I get the same, ie. 11.8M.

 

[Ben Murray]

well i get 11.7647, so youse correct.

That makes me feel better, the engineer has been told the answer is D!?

 

[Julie.]

You are both correct, which answer did he go for?

Is this for the on-line exam aspect?

When carrying out work, I would use the calculation just as you have, but in the exam as it's multiple choice I would just use logic.

It can't be bigger than the smallest measurement (so nothing greater than 20M), and can't be anything lower than a factor of 2-3 lower (even if there were two 20M - in parallel these would be 10M so much less than that couldn't be the case), which usually narrows it down to only one answer.

The mock exam questions can often be complete nonsense!

 

[DPG]

Answer D is completely wrong by several orders of magnitude.

 

[Julie.]

Oops, cross posted, so there are several more correct.

It must be a bad question, it isn't 168ohm!

 

[Ben Murray]

Answer D is completely wrong by several orders of magnitude.

He didn't he stuck to his guns and wrote 11.76M! Good lad haha

 

[Andy78]

Your calcs are right. D is way out.

 

[Ben Murray]

Hi, I have an engineer who is completing his 2391-52 exam tonight and he has been told one of his mock questions was wrong, I have completed the working out myself and came up with the same answer as him, so either the paper is wrong or something has literally left my mind!!

"Determine the overall insulation resistance where the individual readings for four circuits are: 200M ohms, 100M ohms, 50M ohms & 20M ohms"

1/RT = 1/R1 +..... you get the picture.

1/200 = 0.005
1/100 = 0.01
1/50 = 0.02
1/20 = 0.05
Total = 0.085
1/0.085 = 11.76470588235294 (11.76 rounded)

The 4 multiple choice options are:
a) 1.68M ohms
b) 16.8M ohms
c) 16.8 ohms
d) 168 ohms

Please re-educate me and tell me where I am going wrong!

Just so everyone know's (I don't think this is breaking any rules) this is question 12 from Mock paper 2391-52 paper 2

 

[DPG]

Make sure it gets reported so they can correct it.

 

[Andy78]

Just so everyone know's (I don't think this is breaking any rules) this is question 12 from Mock paper 2391-52 paper 2

Is the mock paper a city and guilds release though ? Usually they just get created by training centres and passed about.

 

[Ben Murray]

Is the mock paper a city and guilds release though ? Usually they just get created by training centres and passed about.

I will ask him to mention it. I would be interested to see what the test centre says!

 

[Julie.]

I just checked on the C&G website, there is only one mock test for 2391-52, and question 12 is not this (anyone can access these)

Are you sure it's not a centre's version, or from spxxxxfaxxxts website, these are generally wrong, - questions from earlier editions which are no longer applicable to downright wrong!

 

[snowhead]

Ask the engineer to ask whoever says it's 168 ohms to do or show the calculation that gets to that answer.

Computer says, or that's what's in the book or on the sheet is not an acceptable answer.