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Power-User

Join Date: Jul 2011
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Zero Sequence CT

04/13/2017 10:06 AM

Hello all, i have seen in case of low voltage (400V) motors that some of them have separate zero sequence CT and a zero sequence relay for ground fault protection, while other motors have only main circuit breaker as total fault protection. i cannot understand why some motors have independent zero sequence CT+zero sequence relay for ground fault protection of motor. what does the international standards say about this problem? should we install a dedicated zero sequence CT + zero sequence relay when the motor rating exceeds a certain KW? i am talking only about low voltage motors.

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#1

Re: Zero sequence CT

04/13/2017 10:19 AM

International standard???!!! There is no such thing as an electrical international standard. Why is there no standard, because the power distribution voltages, frequencies and grounding techniques are different. Sometimes the basis for the difference is just legacy issues and sometimes it is to just be different from "the fools" on the other side of your border.

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Guru

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#2

Re: Zero sequence CT

04/13/2017 10:33 AM

International standards dictate big-arse breakers for giant-spinning-pizza-oven motors.

The sequence is:

  • Put blob of pizza dough in oven centre
  • Spin-up oven for one minute at 300 rpm
  • Spin-down oven.
  • Add grated cheese, tomato sauce and other ingredients per customer order
  • Turn on heat
  • Let bake at 230° C for 10 minutes
  • Turn off heat
  • Remove pizza from oven
  • Deliver to customer
  • Repeat

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Power-User

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#6
In reply to #2

Re: Zero sequence CT

04/13/2017 11:45 AM

i understood the meaning of " bullshit" after reading your text

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#7
In reply to #6

Re: Zero sequence CT

04/13/2017 11:53 AM

Really? At this late date you're only now grasping the meaning of the term? If you'd read your own posts all these years, you would have understood the term ages ago.

Coolyar, you are dangerously incompetent and should be forbidden to even approach electrical gear.

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#8
In reply to #7

Re: Zero sequence CT

04/13/2017 11:54 AM

Now, now. Play nicely.

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#9
In reply to #8

Re: Zero sequence CT

04/13/2017 12:46 PM

Unless CY is trolling for fun this is not a game.

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#3

Re: Zero sequence CT

04/13/2017 10:44 AM

Ground fault protection is not there to protect equipment.

It is there to protect people.

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#4
In reply to #3

Re: Zero sequence CT

04/13/2017 10:58 AM

I know of a better way to protect both equipment and personnel. Coolyaar should never be allowed to make any electrical engineering decisions.

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#5
In reply to #4

Re: Zero sequence CT

04/13/2017 11:14 AM

That should be codified as an international standard.

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#10
In reply to #3

Re: Zero sequence CT

04/14/2017 12:52 AM

Don't worry PWS, he's 5400 miles away. We should be safe.

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Power-User

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#11
In reply to #3

Re: Zero sequence CT

04/14/2017 4:11 AM

ofcourse

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#12

Re: Zero Sequence CT

04/14/2017 11:12 AM

As PWSlack remarked, the ground fault protection prevent people to be in contact with a exposed energized metallic surface.

The ground fault protection job is to disconnect the supply line if the duration with respect the voltage level is more than permissible.

If the short-circuit fault current is sufficient elevated in order to disconnect 231 V in 0.4 sec, for example, then a circuit breaker will do the job. If the ground fault current will be less than the short-circuit current which put in operation the protection you need an other means to prevent a dangerous touch -or step- potential.

IEC 60364-1 -for instance- sais:

131.2.2 Protection against indirect contact

Persons and livestock shall be protected against dangers that may arise from contact with exposed-conductive-parts in case of a fault.

This protection can be achieved by one of the following methods:

– preventing a fault current from passing through the body of any person or any livestock;

– limiting the fault current which can pass through a body to a value lower than the shock current;

– automatic disconnection of the supply in a determined time on the occurrence of a fault likely to cause a current to flow through a body in contact with exposed-conductive-parts, likely to cause a current to flow through a body in contact with exposed-conductive-parts, where the value of that current is equal to or greater than the shock current.

There is a safe voltage against duration [according to IEC 61936-1 Standard] :

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#13

Re: Zero Sequence CT

04/14/2017 1:12 PM

The decision for this protection is all economic.

Ground fault protection for motors is to prevent or reduce iron damage due to faults within the stator of the typical induction motor. The motor is much cheaper to repair if the laminations are not damaged.

Zero sequence relays are more sensitive than residual current (vector sum of 3 CT phases connected in wye) relays, so provide earlier detection of a fault, reducing damage.

Typically, a ground fault relay sensing residual current of (3) 1500/5 current transformers will reliably trip at 1500A of ground current, or more.

Your 50/5 Zero Sequence current transformer, connected to some protective relay, may trip reliably between 40A of ground current, or more, highly dependent on the actual CT and the protective relay combination, which are usually tested together.

If your power system is ungrounded, high resistance grounded, or solidly grounded, then this method of ground fault detection will either not work, or be marginally effective, since the high current associated with solidly grounded systems will do significant damage to the motor before either type of ground fault protective device can clear the fault.

If you have a medium resistance grounded system, usually on 2.3kV or 4.16kV systems in the US, then the cost and complexity for the extra equipment is justified, considering the repair cost for the larger motors operating at the higher voltage.

The charging current of higher voltage systems usually prevent the implementation of medium resistance grounding.

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Users who posted comments:

7anoter4 (1); Andrew Westman (3); coolyaar (2); PWSlack (2); redfred (3); rwilliams (1); TonyS (1)

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