94/5 questions. calculations and clarification welcome.

[rustynails]

1) A length of conductor has a resistance of 0.6 ohm. State the expected value of resistance of a similar conductor of
- The same length and twice the csa
- The same csa and half the length
- Twice the length and half the csa
2) State, apart from loose connections, three circumstances that would result in an increase in conductor resistance.
3) For a ring final circuit, state
- The reason why phase to neutral readings at each socket would increase towards the mid point of the ring then decrease again
4) To what IP code should the following comply?
- An intermediate barrier inside an enclosure
5) State why a test on an rcd must not be conducted before a loop impedance test
Scenario
A 15-year-old dry-powder coating factory is situated adjacent to an 11kV/415/240V Regional Electricity
Company substation. The earthing system is T.N-C-S with an external earth fault loop impedance (Ze) of
0.012Ω and a prospective fault current of 20 kA.
The dry-powder coating process involves cleaning metallic components in an acid bath, washing off with water
sprays, drying with hot air blowers, spraying with electrostatically charged powder and finally baking in a
tunnel oven.
The existing installation, which are all radial circuits, is wired with (p.v.c.) conductors in metal conduit and
trunking. Six new lighting circuits supplying high-bay 300W SON luminaires have been installed using a
similar wiring system to the existing installation. The new circuit breakers are fed from a metal clad
distribution board housing BS EN 60898 circuit breakers.
An inspection and test of the whole installation is to be carried out.
6) Describe how an insulation resistance test would be carried out on a the new lighting circuit
7) Explain why it would be appropriate to use BS3036 fuses in a distribution board at the origin of this installation
8)An enclosure has been constructed on site to house electrical equipment, which may be exposed to water.
State the IP codes to which an the enclosure must comply as a minimum requirement?

I'd like to see what others answer for these to verify my own answers because I have no answers only my own...

 

[Simonslimline]

1) A length of conductor has a resistance of 0.6 ohm. State the expected value of resistance of a similar conductor of
- The same length and twice the csa
- The same csa and half the length
- Twice the length and half the csa
2) State, apart from loose connections, three circumstances that would result in an increase in conductor resistance.
3) For a ring final circuit, state
- The reason why phase to neutral readings at each socket would increase towards the mid point of the ring then decrease again
4) To what IP code should the following comply?
- An intermediate barrier inside an enclosure
5) State why a test on an rcd must not be conducted before a loop impedance test
Scenario
A 15-year-old dry-powder coating factory is situated adjacent to an 11kV/415/240V Regional Electricity
Company substation. The earthing system is T.N-C-S with an external earth fault loop impedance (Ze) of
0.012Ω and a prospective fault current of 20 kA.
The dry-powder coating process involves cleaning metallic components in an acid bath, washing off with water
sprays, drying with hot air blowers, spraying with electrostatically charged powder and finally baking in a
tunnel oven.
The existing installation, which are all radial circuits, is wired with (p.v.c.) conductors in metal conduit and
trunking. Six new lighting circuits supplying high-bay 300W SON luminaires have been installed using a
similar wiring system to the existing installation. The new circuit breakers are fed from a metal clad
distribution board housing BS EN 60898 circuit breakers.
An inspection and test of the whole installation is to be carried out.
6) Describe how an insulation resistance test would be carried out on a the new lighting circuit
7) Explain why it would be appropriate to use BS3036 fuses in a distribution board at the origin of this installation
8)An enclosure has been constructed on site to house electrical equipment, which may be exposed to water.
State the IP codes to which an the enclosure must comply as a minimum requirement?

I'd like to see what others answer for these to verify my own answers because I have no answers only my own...

Is this a typo? Should it read, inappropriate?

 

[rustynails]

hmmm that was my initial thought. I found this paper in a drop box of other papers I was given access to by another forum member for 94/95 resources. In a folder called 2394-95 exam Q&A I found 2395samplepaper-A question 26.B...

 

[Silly Sausage]

Come on Rusty, you know the rules, show us your answers first.

Constructive criticism will rapidly follow!

 

[happysteve]

Come on Rusty, you know the rules, show us yours first.

Constructive criticism will rapidly follow!

Corrected that for you

Round our way, they say, "You show me yours, I'll show you mine..."

 

[Silly Sausage]

Believe me, you wouldn't want to see mine first.
I only suggest answers first so it'll it give me an idea to find the right answers myself!!!

 

[rustynails]

Fair enough...

1)
a. 1.2
b. 0.3
c. 0.9
2)
1. Ingress of water
2. Increase in conductor temperature
3. An addition in length to the circuit
3) The incoming Line has been connected with the incoming Neutral at the distribution board and vice versa
4) IP4X
5) An earth fault loop impedance test verifies all parts of the earth fault loop path verifying that the circuit is correctly wired and connected to earth so that an RCD would operate on the circuit as an RCD relies on a path to earth.
6)

• Seek permission to isolate the installation from the relevant person/s
• Using the safe isolation procedure isolate the installation
• Using an Insulation Resistance tester check the instrument is not damaged, check the batteries are in good condition, check the instrument is calibrated and check the leads comply with HSE GS38
• Disconnect any current using equipment such as high bay luminaires, any neons, any electronic equipment that might be effected by the test voltage
• Check all light switches are closed and lamps have been removed
• At the distribution board test at 500V between L-N, L-E and N-E
• Operate any two way switching and test again to include any strappers
• Reading should be greater than 1Mohm
• Any readings below 2Mohm require further investigation, the testing of individual circuits would be recommended.
• If certain equipment cannot be disconnected it is acceptable to test L and N joined together to Earth at 500v, readings should be greater than 1Mohm

7) Due to the use of the installation, spraying with electro statically charged powder, ceramic based fuses have less mechanical parts acceptable to damage from such environmental influences.
8)IP2X, IP4X

 

[Simonslimline]

1) A length of conductor has a resistance of 0.6 ohm. State the expected value of resistance of a similar conductor of
- The same length and twice the csa 0.3 ohms
- The same csa and half the length 0.3 ohms
- Twice the length and half the csa 2.4 ohms
2) State, apart from loose connections, three circumstances that would result in an increase in conductor resistance. earth fault, short circuit, ambient temperature.
3) For a ring final circuit, state
- The reason why phase to neutral readings at each socket would increase towards the mid point of the ring then decrease again.Length of conductors is longest at the midpoint.
4) To what IP code should the following comply?
- An intermediate barrier inside an enclosure IPXXb or IPX2
5)
State why a test on an rcd must not be conducted before a loop impedance test. To check the requirements have been met to prevent potentially dangerous voltages on exposed conductive & extraneous conductive parts.
Scenario
A 15-year-old dry-powder coating factory is situated adjacent to an 11kV/415/240V Regional Electricity
Company substation. The earthing system is T.N-C-S with an external earth fault loop impedance (Ze) of
0.012Ω and a prospective fault current of 20 kA.
The dry-powder coating process involves cleaning metallic components in an acid bath, washing off with water
sprays, drying with hot air blowers, spraying with electrostatically charged powder and finally baking in a
tunnel oven.
The existing installation, which are all radial circuits, is wired with (p.v.c.) conductors in metal conduit and
trunking. Six new lighting circuits supplying high-bay 300W SON luminaires have been installed using a
similar wiring system to the existing installation. The new circuit breakers are fed from a metal clad
distribution board housing BS EN 60898 circuit breakers.
An inspection and test of the whole installation is to be carried out.
6) Describe how an insulation resistance test would be carried out on a the new lighting circuit ​disconnect any voltage sensitive equipment from the circuit, also any components that will give inaccurate readings like capacitors etc. The lamps & control gear would need to be disconnected also. Test between live conductors, and live conductors and earth (cpc still connected to the means of earthing). Test at 500Volts. Record values obtained. If the lighting is not accessible to remove the lamps and gear then it is acceptable to perform the test with the relative switch in the open state.
7) Explain why it would be inappropriate to use BS3036 fuses in a distribution board at the origin of this installation.. Typo i think. The PFC is too high for these type of fuses.
8)An enclosure has been constructed on site to house electrical equipment, which may be exposed to water.
State the IP codes to which an the enclosure must comply as a minimum requirement? IPX4, IPX5, IPX6

I'd like to see what others answer for these to verify my own answers because I have no answers only my own...

Hopefully not too far off with these.

 

[rustynails]

Question 1, all parts, is correct.
Interestingly how did you come up with 2.4 for part 3?

Question 7 I didn't read it slowly enough... IN appropriate of course your answer is correct PFC is too high for these type of fuses.

I don' have any official answers to the rest yours sound good tho.

 

[Simonslimline]

Question 1, all parts, is correct.
Interestingly how did you come up with 2.4 for part 3?

Question 7 I didn't read it slowly enough... IN appropriate of course your answer is correct PFC is too high for these type of fuses.

I don' have any official answers to the rest yours sound good tho.

Resistance increases with length. Resistance will also increase if the cross sectional area of the cable decreases.

So the cable has doubled in length, and also halfed in cross sectional area.

Double the length = 2 x 0.6 = 1.2 ohms
Half the csa = 2 x 1.2 = 2.4 ohms

Been a while since using any of the theory so hopefully not forgotten it.

 

[Richard Burns]

Simonslimline has given some good answers.
I agree with his answers for number 1.

For number 2, I am slightly lost also, the answers Simonslimline has given are correct, a high current flowing through the conductor or a high ambient temperature will raise the conductor temperature and so generate a higher resistance in that conductor, however fault current to earth or another live conductor is in both cases just a higher current and I cannot think of another cause at this level of question.

Number 3 obviously in a ring the total length of the conductors is the same at each point in the ring (bar spurs), however when measuring you are measuring two resistors in parallel (the long leg and the short leg).
For parallel resistors the total resistance is lower than the lowest individual resistance.
When measuring with one leg very short its resistance is low and so the overall resistance is low, the highest resistance that you can encounter is where both legs are the same length i.e. the middle of the ring.

Number 4 As simonslimline although I think there was a typo with IPX2 and it should be IP2X (solids not liquids).

Number 5 pretty much as Simonslimline, if you have not verified that there is an earth present and correctly and safely connected then when introducing a voltage on the earth during the RCD test the earth conductors may rise to a dangerous potential.

Number 6 As simonslimline as this is pretty much straight from GN3.

Number 7 As simonslimline

Number 8 since it states in the description that water sprays are used in the process and that the enclosure may be exposed to water then I would expect a minimum of IPX5, as it is unlikely that heavy directed sprays would be aimed at the enclosure (or I hope not), but it may be subjected to tangential spray. I am being a bit over cautious here because this could well be considered splashing water (IPX4), but in the context of the question I expect they wish to see IPX5 as the answer.
Additionally as it is an enclosure the minimum of IP4X on horizontal accessible top surfaces and IP2X on sides and bottom is required.
Therefore a minimum IP rating would be IP44, (possibly IP45) this covers the questions statement of IP codes.

 

[Simonslimline]

Yes indeed a typo and it should of read IP2X. Well spotted.

As for the ring circuit, perhaps an overload on the circuit as another answer?