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alexjan
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In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/17/2018 6:54 PM

hi,in tn-s system if input side of a consumer panel has only 4 cables, 3 numbers of them are L1, L2 and L3 (phases) but the forth is what?
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JRaef
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#1

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/17/2018 7:11 PM

Neutral

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alexjan
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#2
In reply to #1

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/17/2018 7:16 PM

thanks a lot, why? tns must have 5 cables. why have four? and why the forth is N? i think it must be PE. why PE is wrong?
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#3
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Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/17/2018 7:38 PM

It's more likely to be a neutral conductor than an earth. Are you sure the system is not TN-C or TN-C-S, either of which would likely not have a separate incoming protective earth.

I'm assuming that you have positively determined that the other three conductors are indeed three phases and not just two phases ie, two phases plus neutral plus earth making 4 wires.
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#5
In reply to #3

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/17/2018 10:01 PM

Another thought here is that you mention it is the last consumer.

Sometimes in TN-S systems, especially where the protective earth is provided via a metal sheath of the supply conductors but also in overheads where the impedance of the PE conductor is too high to ensure adequate protection, the consumers at the end of the line are sometimes converted to a TN-C-S, TT or even a pseudo MEN system with a local ground stake and bond to buried water pipes to improve earth characteristics.

This may be what you are seeing.
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PWSlack
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#13
In reply to #2

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 3:13 AM

<...why PE is wrong?...> It would only have four if it were a TN-C-S, or an unearthed, installation. And if it is wrong, the act of engaging a qualified local Electrician to put it right is overdue.

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#4

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/17/2018 7:54 PM

Depends on which type of system you are installing....

..."TN system

In TN systems, one point is connected directly to earth and the exposed conductive parts of the electrical installation are connected to this point via protective earth conductors.

There are three types of TN systems, differentiated on the basis of the arrangement of the neutral and protective earth conductors:

  • TN-S: The protective earth conductor is separate throughout the system.
  • TN-C: The neutral and protective earth conductors are combined in a single conductor throughout the system
  • TN-C-S: Neutral and protective functions are combined in a single conductor in a part of the system."...

https://www.bender-apac.com/know-how/technology/it-system/comparison-of-system-types

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alexjan
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#8
In reply to #4

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 2:09 AM

So many thanks, the system is tn-s and not tn-c. I know if the last load is Squirrel cage motor, it need no N and need only 3phases+PE then in output of the last panel we see 4 wires. But in this case input of the last panel has 4 wires.

is this right: that if our load is Squirrel cage motor, even the input wires into panel will be 4 whitout N (only 3phases+PE)? (i know ouput of a panel can have 4 wires but input of a panel in tns system can have 4 wires?)

Another case that i think is when cable section is greater than 10 mm^2, there is no seperating PEN into PE+N in main panel and we can seperate in the last panel so the forth is PEN. But the system is tn-s. then in this case could we name the system tn-s not tn-c or tn-c-s?
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#11
In reply to #8

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 3:08 AM

<...the system is tn-s and not tn-c...> Nonsense.

If in doubt, consult a qualified local Electrician.

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#6

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/17/2018 10:46 PM

yes, yes, YES. Three questions in a row today that I can answer with the same link.....

Here is the main book that is pretty much the comprehensive lightning protection design guide (in my opinion anyway). It includes the different electrical systems and their connections and what they are for, and its free.

DEHN

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#7

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 12:04 AM

If the supply is from a dedicated transformer the N→E link will be in the first incoming panel. If so you will see only four conductors.

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alexjan
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#9
In reply to #7

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 2:14 AM

thanks, but this in this case input of a last panel has 4. please see my last reply above
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alexjan
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#10

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 2:17 AM

So many thanks, the system is tn-s and not tn-c. I know if the last load is Squirrel cage motor, it need no N and need only 3phases+PE then in output of the last panel we see 4 wires. But in this case input of the last panel has 4 wires.
is this right: that if our load is Squirrel cage motor, even the input wires into panel will be 4 whitout N (only 3phases+PE)? (i know ouput of a panel can have 4 wires but input of a panel in tns system can have 4 wires?)
Another case that i think is when cable section is greater than 10 mm^2, there is no seperating PEN into PE+N in main panel and we can seperate in the last panel so the forth is PEN. But the system is tn-s. then in this case could we name the system tn-s not tn-c or tn-c-s?
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#12
In reply to #10

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 3:10 AM

<...the system is tn-s and not tn-c...> Nonsense.

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alexjan
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#14
In reply to #12

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 3:31 AM

this was an exam question. and answers were:

1)N

2)PE

3)PEN

4)none
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PWSlack
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#15
In reply to #14

Re: in tn-s system, input of a electrical panel of the last consumer has only 4 cables

05/18/2018 9:53 AM

...then see the Wikipedia page on Earthing, #11⇑.

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#16

Re: In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/18/2018 11:37 AM

The question is sdrawkcab. It depends upon what the nature of the fourth conductor is as to the name of the <...system...>, and not the other way round.

What are the colo(u)rs of the insulation on each conductor?

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alexjan
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#17
In reply to #16

Re: In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/18/2018 1:47 PM

thanks, the question has said nothing about color or section.

only said the system is tn-s and the input of the last consumer panel has 4 conductors. which one is right about forth conductor?

1)N

2)PE

3)PEN

4)none
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Oomborrie
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#18
In reply to #17

Re: In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/19/2018 8:09 AM

Trick question of which the lecturer also do not know the answer as according to this---

"TN−S PE and N are separate conductors that are connected together only near the power source."

That cable can only be Neutral if the earth cable is detectable as all supplies L1 L2 L3 N and E must come via an insulated conductor.

There must be 5 cables in a TN-S system as that is what define the system. There can be no right answer to the question.

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#19

Re: In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/20/2018 9:52 PM

So, not only are we doing homework questions, we are fielding test questions too??

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Oomborrie
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#20
In reply to #19

Re: In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/21/2018 7:31 AM

Dj95401, this young man posted here because he was taught and researched this specific electrical setup and he got asked a question about 4 conductors that do not fit the information anywhere to successful answer that question. He feels cheated because the institution and the educated faculty personnel made their own new rule, not supported by any other documentation and he may have lost the chance for a bursary and here it is not his fault. We can not condone incorrect exam questions and answers. A TN-S instalation have 5 separate insulated conductors from the Source/supplier to the user and that is not debatable. How you distribute it after your incomer is up to your instalation requierments but up to your incomer it is 5 separate insulated conductors.

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#21

Re: In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/21/2018 7:49 AM

If documentation and observation disagrees, then make no assumptions. Throw away everything you take for granted, trace the cables back to the source.

For all we know, the fourth could be a single-wire earth-return telephone. The type of "electricians" that wire up things while ignoring schematics and doumentation can produce quite some "creative solutions" at times.
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#22

Re: In tn-s System, Input Of A Electrical Panel Of The Last Consumer Has Only 4 Cables

05/21/2018 3:50 PM

<...the forth...>.

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