2391-10 Written Exam - 01/12/11

[Des 56]

2 supply characteristics that can only be found out via enquiry to DNO

Ian I posted on this in another thread to the question above

The Pfc and the Ze can be measured or by enquiry
Frequency and voltage is nominal ,so these 2 cant be known by measurement only enquiry,city and guilds typical vague question

 

[Banjo]

2 supply characteristics that can only be found out via enquiry to DNO

Ian I posted on this in another thread to the question above

The Pfc and the Ze can be measured or by enquiry
Frequency and voltage is nominal ,so these 2 cant be known by measurement only enquiry,city and guilds typical vague question

Wouldn't the type and rating of the fuse be one aswell seeing as you can't pull the fuse to 100% know the tye and rating?

 

[Scotchydog]

what was it then got to admit i put earthing arrangements, frequency voltage

 

[Former Sponsor TM]

Guys, what about Ra of the electrode at the supply transformer??

 

[firemanowl]

If you look on the supply characteristics section on the test sheet in the fine print it says that you can only get nominal voltage and frequency from the supplier, just noticed it now. There was a question in Part B about the supply fuse or did I make that up?

 

[pfjt]

Can anybody the wording of q1

 

[pfjt]

Remember

 

[firemanowl]

I'm sure Q1 was what do you have to verify on an initial verification and Q4 was why do you carryout an initial verification or something like that. Seemed an overkill on the initial verification questions?

 

[tony mc]

Guys, what about Ra of the electrode at the supply transformer??

What was the question?

I should imagine no more than 21 ohms!

 

[Banjo]

Guys, what about Ra of the electrode at the supply transformer??

I can't remember that question? :S

 

[pfjt]

There wasn't a question on earth electrode resistance was there

 

[brav]

Wot about the Supply Transformer to MET for tt, tns, tncs? Did u start at Transformer i.e. transformer, supplies Line conductor.. etc, or start at MET i.e. MET, installations Earthing Conductor...?

 

[leonard]

Yes Banjo that's what i did the only supply characteristics that you cant measure.

 

[pfjt]

I don't remember this question how was it asked

 

[Danm1877]

The volt drop answer was 0.38x30Ax1.2=13.68v. Which is over 11.5v and exceeds the maximum volt drop. But I couldn't work out if the roller shutter was in a private or public installation as it was in a retail outlet but not part of the shop??????! This would change the max volt drop.

 

[cr0ft]

Our tutor told us the day before the exam that you no longer multiply by 1.2 now. Hope he was right or I am wrong on that one!

 

[Marnik]

Votage drop calculation formula= R1 + Rn x 1.2 x Ib
So in this case = 0.38 x 1.2 x 30
Voltage drop = 13.68v = 5.95 % DOES NOT COMPLY!!

 

[leonard]

hi marnik have you maid it up because the formula for calculating the volt drop is Vd=(R1+Rn)xId
and on GN-3 said that on circuits supplying motors with high inrush starting current is allowed higher voltage drop and 1.2 factor is not mentioned.

 

[i=p/u]

well im getting taught at present and last year level 2 they didnt mention that 1.2 factor for voltage drop

 

[malcolmsanford]

The volt drop answer was 0.38x30Ax1.2=13.68v. Which is over 11.5v and exceeds the maximum volt drop. But I couldn't work out if the roller shutter was in a private or public installation as it was in a retail outlet but not part of the shop??????! This would change the max volt drop.

???????????

 

[Danm1877]

There is different % of allowed voltdrop from a electrical installation that is in a public area than one it is a private area. Hope this solves your ???????????

Sounds like we are all being taught something different!

 

[pfjt]

It's not areas it's distribution networks there are public distribution and private distribution ie if a factory has its own transformer in its own substation or public is if you are off a standard substation

 

[malcolmsanford]

There is different % of allowed voltdrop from a electrical installation that is in a public area than one it is a private area. Hope this solves your ???????????

Sounds like we are all being taught something different!

I suggest you have another think about that.

As far as I was aware the only deviation in the volt drop values allowed in the BS 7671-2008 concerns motors and is regulation 525.4 in the BRB. Now as the C&G 2391 based on the BS 7671 perhaps that is what you mean when referring to a roller shutter.

 

[NMc]

Q1 was something like this - an inspection of an installationis being done as part of an intial verification. What 3 conditions must be verified?

Q4 purpose of initial verifcation as per GN3

 

[NMc]

In a way you are both right App 12 has two for a public (DNO) system and two for a private supply. 525.4 is as stated in quote

The question as I remember was something like - a single phase motor is running slow and gets hot and it is believed it is a volt drop issue
1. calc vd if L1 & LN is 0.38 and motor load is 30A
2. does vd above meet regs
3. one course of action to be taken if vd is too high and why

I suggest you have another think about that.

As far as I was aware the only deviation in the volt drop values allowed in the BS 7671-2008 concerns motors and is regulation 525.4 in the BRB. Now as the C&G 2391 based on the BS 7671 perhaps that is what you mean when referring to a roller shutter.

 

[malcolmsanford]

Yes your quite correct there is different value in the appendix for private supplies such as generators and I did forget this , so the C&G may have thrown that one in, but never known them to use it before.

Thanks for reminding me.

 

[LostInAcademia]

What test voltage did you put for the 25v question?

should be 500V as question did not state it was a SELV or PELV circuit.

 

[Jurasic Spark]

Votage drop calculation formula= R1 + Rn x 1.2 x Ib
So in this case = 0.38 x 1.2 x 30
Voltage drop = 13.68v = 5.95 % DOES NOT COMPLY!!

1.2 is the multiplying factor to convert the resistance of a copper conductor at a temp' of 20 degrees C to a temp' of 70 degrees C.

The question gave resistances of R1 and Rn but did not say at what temperature. So why are you using 1.2 ??

 

[Danm1877]

1.2 is used because you have to take maximum operating temp of the cable when making this calculation. Otherwise when the circuit is in use and the temp of the cable rises the calculations and current carrying capacity will change. Worst case Scenario. Otherwise it would comply when not in use but possibly not when in use.

 

[Jurasic Spark]

1.2 is used because you have to take maximum operating temp of the cable when making this calculation. Otherwise when the circuit is in use and the temp of the cable rises the calculations and current carrying capacity will change. Worst case Scenario. Otherwise it would comply when not in use but possibly not when in use.

1.2 is used to convert a 20 degree resistance reading to a 70 degree resistance reading. See p105 of GN3:

For examplea reading taken at 20 degrees C could be converted to an equivalent reading at 70 degrees by determining the correction factor obtained from p105:
(1 + 0.004 (amb temp - 20))

= 1 + 0.004 (70 - 20)

= 1 + 0.004 x 50

= 1.2 So this is the multiplying factor to convert 20 degrees C to 70 degrees C.

BUT THE IMPORTANT THING IS THE QUESTION DID NOT SAYWHAT TEMP THE READINGS OF R1 & Rn WERE TAKEN AT.

 

[leonard]

should be 500V as question did not state it was a SELV or PELV circuit.

hi mate ,up to 50v is ELV so should be tested at 250vDC

 

[Danm1877]

Hi guys. Hope you all have your results. How did everyone get on?