2391 questions 17 Th edition

[Banburyman]

• the initial values recorded between the ends of individual P and N loops during a ring final circuit test were both 0.6 ohms. If cable is flat twin with cpc state:

reason why P to cpc reading will increase slightly to the mid-point of the ring and then decrease again?

• state the max disconnection time
  • domestic socket outlet
  • site hut lighting on construction site
  • 110V reduced low voltage system
  • socket outlet which are part of a TN system where livestock is to be kept
  • 230V fixed equipment on a construction site
  • general socket outlet
  
  
• state three reasons for the use of 500mA RCD in a installation

I would be grateful foe any right answers

 

[scotsparky]

Q1 the Zs will increas slightly as its the furthest point slightly increased resistance on CPC. However when doing it for real watch out for spurs they will give you and odd reading.

Q2 (i) 0.4
(ii) 0.4
(iii) 0.8
(iv) o.4
(v) why 230 on construction site? RLv on contruction sites. if not 0.4
(iv) 0.4


see page 46 BRB table 41.1 you will find the spectial locations section will point you back to this page.

 

[Banburyman]

State three methods of ascertaining the value of prospective shou\rt current ata the origin of a TNC-S system

by calculation
by enquiry
one more????

Thank you for quick respond

230V fixed equipment on a construction site
"(v) why 230 on construction site? RLv on construction sites. if not 0.4" - this is a question from 2391 test

did you take into account range 120V<U<= 230V (table 41.1) to obtain the disconnection time eg: socket outlet which are part of a TN system where livestock is to be kept = 0,4

 

[scotsparky]

the Ze PFC one is by measurement.

As on a construction site its reduced low voltage from a centr tapped transformer its 55 v on each live conductor the P-E and will be protected by RCd voltage is see page 46 BRB fig 41.1 thast maximum however in reality the rcd will trip within .4

for the livestock one as no earthing system was stated i used highest reading. It its tt then .2

 

[mkmukok]

• the initial values recorded between the ends of individual P and N loops during a ring final circuit test were both 0.6 ohms. If cable is flat twin with cpc state:

reason why P to cpc reading will increase slightly to the mid-point of the ring and then decrease again?

If you crosstest continuity of ring main final circuit with singles and you don`t know wich is outgoing and wich incoming leg, then you may have such readings.

 

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State three methods of ascertaining the value of prospective shou\rt current ata the origin of a TNC-S system

by calculation
by enquiry
one more????

Measured (reg 612.11 & reg 313.1)