Disconnecting phase and neutral 2

[AusLee]

Hello,

Can you please explain this part of the Australian Wiring Regulations:

"Where an item of switchgear is required to disconnect all live conductors of a circuit, it shall be of a type such that the neutral conductor cannot be disconnected or reconnected without the active conductors also being disconnected or reconnected respectively."

Ok, in the case of a single phase circuit: "All live conductors of a circuit" means one one conductor, the phase; and as per this article i should disconnect the neutral also, "using a deivce that allows the neutral and the phase(s) to be connected or disocnnected simulatensously". So i will need a two pole breaker.

In the case of a 3 phase circuit: All live conuctors means all three phases, so i must use a 4P breaker to disconnect the neutral at the same time. This leaves a flaw of being able to disconnect one phase with a subsequent breaker as well.

So they do not use single pole breakers? Am i misunderstanding this requirement? THanks for your help.

 

[rbulsara]

This is not different than any other Codes like NEC or engineering principles.

It is not even mandating disconnection of neutral, but if required ( Note: IF required) N shall be last to disconnet and first to reconnect amnong the group of conductors. Such a case will be switching between two seprarately derived sources.

Or a equipmnet requiring total disconnction. But in no case this prohibits use of single pole or 3 pole breakers on a 3ph 4-wire circuit.

 

[AusLee]

Ahhhh! "All live conductors" so Neutral is a Live conductor. I see - thanks for your answer!

Please one more thing about switching between sources. I specify my ATSs as 4P but i want to ask you this: according to some IEC code - cannot remember which - There shall be a means do permanently disconnect a cable from source in case of a fault.

Therefore, consider this: I have 2 incomings, Generator and Transformer, terminating each in a contacotr or circuit breaker with electromechanical interlock between the two. Now the output of the two breakers are shunted together and only one cable goes out to the load.

Suppose this cable faults while supplied from transformer, then the circuit breaker from the transformer side will trip. Now when the public network fails, the generator will run and its breaker will close on a faulty cable and will trip soon after.

According to IEC, there should be a third breaker which feed the load cable and it is that breaker that must trip in case of fault. Is that Really true? the 3rd breaker can incurr $20,000 more for a 2500A PACB with cubicle!

One last thing about ATS but in the same context, is the ATS required to have a Zero position? so it is Either Manual mode or Autmatic mode of Off mode, nothing out, for maintenance reasons. I ask this because i just recieved an ATS from Kohler with and inverter switch, no off position and want to ask if this needs to be complemented by something.

Thanks agian.

 

[itsmoked]

Would seem to me you could condition the generator to not start unless there was no utility as opposed to starting on loss of voltage at the load. Thereby avoiding your situation.

 

[jghrist]

Note that the regulations state that

...the neutral conductor cannot be disconnected or reconnected without the active conductors also being disconnected or reconnected respectively.

They do not say that the active conductors cannot be disconnected without disconnecting the neutral.

When you have a transformer and a generator as sources to an ATS, and the neutral is grounded on both, then the neutral should be switched so that you do not have two grounds on the neutral. If the neutral is not grounded at the generator, then the neutral should not be switched.

 

[AusLee]

itsmoked: yes, i asked the supplier to make out a Procedure to maneuvering the equipment, thereby stating that in order to completely shut down the power, the generators shall be put into Manual mode and the ATS switched manually to the generator side. One small problem: the Generator is in another room and there is no mean to lock that particular switch in the Manual position, one has to lock the generator room. If i can find something in the litterature about a Zero position then i can order one more cubicle and keep what i am getting now a temporary solution.

What about the IEC thingy and the need for a third device, will NEC authorize an ATS with only 2 interlocked switches?

jghrist: i will look it up in NEC, if the generator's neutral is earther, then it iwll be called a "Separately derived source" and hence the 4P breaker. When you say "should not be switched" you mean: it is wrong that i switch it or it is ok if i do no switch it?

 

[rbulsara]

Coordination is a separate issue than application of an ATS.
ATS is just a switch. If interlocked circuit breakers are used as ATS, as dpc said either you can condition the ATS not to transfer upon a fault or take your chances.

In the worst case the gen breaker will open, normally generator short circuit currnt will be less than that of a utility. A Properly rarted (SCCR) breaker should not blow up even if closing on a fault.

A 4P ATS is acceptable, Codewise, in all cases. A 3P ATS requires that both sources are grounded at only one point. Go to Caterpillar website and locate a paper on this issue and read it up.

 

[jghrist]

If the generator is not a separately derived source, that is, the neutral is not grounded at the generator, then you should not switch the neutral. If you did, then when you switched to the generator, your neutral would be ungrounded.

 

[rbulsara]

jgrihst is right...

What I intented to say but did not come out right was that 4P can be made to work in all conditions. Most you do is ground both sources if not already. A 3P will requires careful grounding and ground fault protection schemes.

3P ATS are more common that 4P, but both have their places.

 

[waross]

Re switching neutrals. I don't agree with switching neutrals.
I have connected a lot of transfer switches, large and small, manual and automatic. I have never had a problem with a solid neutral, but have seen quite a few problems when a neutral goes open.
I have always been able to devise a way to connect grounds to comply with codes and leave the neutrals solidly connected. Any switch may fail, and when it is the neutral section of the switch that fails, expect expensive problems.

 

[ScottyUK]

waross,

Have you ever connected a system where the generator operates in parallel with the utility? If one of the neutrals is not switched, then current will flow in the earth path as well as the neutral conductor. This is a code violation in most countries that I am aware of, and it can cause operational problems with differential protection and E/F protection mis-operating. If you are able to ground your generator neutral at the transformer neutral earthing point then you arguably can avoid the problem but this is an option in very few instances.


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Your body might be a temple. Mine is an amusement park...

 

[waross]

Take a look outside your house. In many places in the world, a common neutral is used for both the primary distribution and for the secondary. In other places the neutrals are interconnected. Many jurisdictions ground the neutral at each distribution transformer. Many jurisdictions require the neutral to be grounded at each consumers service. In many jurisdictions, the ground leads from primary lightning arrestors are bonded to the neutral on their way to the ground rod. Take a walk down the street a count how many grounds are connected in parallel with your own residential service. Ground currents may not be as great or as serious a problem as you think.
Re differential tripping on ground currents. If you can't connect a generator with a solid neutral so as to satisfy code, you may also have trouble arranging your c/t's to provide proper protection.

Generators in parallel with the utility, yes. An equipment ground for the generator frame as required by code for protection of personel and a system ground as required by code.
At times you may have to run the grounding cunductor through a c/t to assure proper connection, but it's usually easier to plan the connection in a different location originally.
I repeat,
"I have always been able to devise a way to connect grounds to comply with codes and leave the neutrals solidly connected."
But hey, apart from this small disagreement, I respect and agree with most of your posts.

 

[ScottyUK]

Hi waross,

That practice is known as protective multiple earthing, or PME, and is strictly the province of the distribution utility in the UK. It is explicity not allowed for installation wiring after the utility service entrance, where separate neutral and earth conductors must be run. Connection of embedded generators on the UK's public LV network is not something I'm at all familiar with so I can't do other than speculate how these are protected. My comments were directed at generation embedded on a private LV network without PME. I should have stated this - sorry, I completely forgot about PME systems.

Can you expand a little on

At times you may have to run the grounding cunductor through a c/t to assure proper connection, but it's usually easier to plan the connection in a different location originally.

The only time I've seen a CT specifically on an earth connection was for frame leakage protection on an archaic MV switchboard. CTs on neutrals, core balance CTs around all live the conductors, individual CTs on each conductor and tied into a diff scheme: all these I am familiar with, but CTs on a ground conductor for LV installations is a new one for me. I'm keen to learn!


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Your body might be a temple. Mine is an amusement park...