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Topic Title: Questionaire from job interview!
Topic Summary: Understanding what the assessor requires?
Created On: 28 March 2013 10:11 AM
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 28 March 2013 10:11 AM
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samwells

Posts: 1
Joined: 28 March 2013

I was asked the following questions in a job interview, and just wondered what you all think the correct answers are?

1. What is the overload setting for a motor having a full load current rating of 100 amps.
a) direct on line setting
b) star delta setting

2. You need to install a cable for a welding machine rated at 45 amps 3 ph. There is a 125 amp 4 pin & earth socket outlet. What materials would you ask for and why?

3. Between a single core and mult core cable which has the greater current carrying capacity?

Any comments would be gratefully received and might help relieve my mind!! Cheers
 28 March 2013 01:48 PM
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Avatar for davezawadi.
davezawadi

Posts: 2816
Joined: 26 June 2002

Those are interesting questions Sam. Consider these guides to the answer, you probably should have thought these and ideally mentioned them out loud:

1. What is the motor overload for? How long does it take to trip? Will a short term over current damage the motor? Is normal speed operation of the motor different depending on the starter?

2. This question is missing a fair bit of important information. Ideally you need to know:
Does the welder use only 2 of the phases or all three?
What is the CPD rating (it may be different to 125A)?
What is the operating environment?
Check that this is the flexible cable fixed to the welder being asked about, not a circuit extension with an additional socket.
Is the welder fused against internal faults itself, if not where do they want such protection?

3 You know this from any of the regs cable tables, but single cores dissipate heat better so have higher ratings than the same size copper in a multicore cable.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 28 March 2013 02:46 PM
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Avatar for OMS.
OMS

Posts: 19898
Joined: 23 March 2004

Originally posted by: samwells

I was asked the following questions in a job interview, and just wondered what you all think the correct answers are?



1. What is the overload setting for a motor having a full load current rating of 100 amps.

a) direct on line setting - 100% of FLC ie 100A

b) star delta setting - depends on where the overload is positioned but usually 58% of FLC - ie 58A


2. You need to install a cable for a welding machine rated at 45 amps 3 ph. There is a 125 amp 4 pin & earth socket outlet. What materials would you ask for and why?

You can only work with the data you have so a minimum CPD In of 45A plus a characteristic to hold welder inrush current plus cable that has an in-service current rating of greater than In for the installed conditions and taking account of the environment

3. Between a single core and multi core cable which has the greater current carrying capacity?

A quick look at any cable table should show you a marginal increase in curent carrying capacity for single core cables compared to multicore cables of identical CSA


Any comments would be gratefully received and might help relieve my mind!! Cheers


regards

OMS

-------------------------
Failure is always an option
 28 March 2013 02:50 PM
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Avatar for rocknroll.
rocknroll

Posts: 8903
Joined: 03 October 2005

1. What is the overload setting for a motor having a full load current rating of 100 amps.
a) direct on line setting
b) star delta setting


Obviously if this is an interview they are looking for a very quick answer to see if you have the 'knowledge' even I remember this one. LOL

a) FLC x the service factor which could be 1.0, 1.15 or 1.25, if the motor has no service factor the general rule of thumb is to go for 1.15 or 115 amps.

b) If the overload is to be placed in th 'line side' then it is the same as DOL, if the overload is to be placed in the 'winding side' then it is 0.58 x FLC or 58 amps.

(Note: if the overload is placed in the line side it is not protecting the motor while in delta as it is 1.732 too high).

Time me thinks for some study and enjoy.

regards

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"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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