Breaker

[Engcan]

Hi Everybody
Happy Holidays !

Here are my two questions;

Q#1, I am a new start in this area so while specifying the rating of a 120 v panel some thing flashed in my mind.

we have a 3 ph supply of 416/240 v and then a 500 VA 240/120 v transformer is connected to a 15 A breaker on this 416/240 v panel, and this transformer willsupply 7 Compact flourscent lights 20 w each(single phase). Now we can load a breaker up to 80 % but in this case the current is only 2 Amp on breaker side. What will happen when there is a overload may be a single line to ground fault. As there is only phase and neutral on secondary side, so no possibility of line to line fault, but do you think, a 15 Amp breaker having a primary loading of 2 Amp and secondary loading of 4.2 Amp will trip on ground fault. Please advise.

Q #2, We have a power distribution project as normally utilities do the power distribution but the local utility has asked the owners to proceed and it is around 2.5 Sq. km area with around 600 homes. Now, we come under the preview of Electrical Safety authority and that is the hard part, they have come up with so many things which utilites will not even think about doing. Here is my question below:

They have asked us to put a load break switch on every pole from where there is a tap to pad mounted transformer, I was wondering, single pole switching leads to ferroresonance, so should I use a three pole or single pole. If I use three pole then the other two feders will go out of service also, but If I use single pole then there might be ferroresonance.
The other part of this question is The ESA asked for an interlocking mechanism with pad mounted transformer and with all the others which are looped and connected to the one which is having a tap from over head line. What does that mean ?

I hope I have tried to make my self very clear. I would appreciate any word from you gurus.

Best Regards,

 

[Engcan]

Hi Gurus,
I woulds appreciate a moment from you guys for answering these questions.
Thanks

 

[buzzp]

I can only address part of your first question. Your talking about two different things in your first question. There is a difference between an overload and a fault. A 15A breaker whos hot line contacts the ground, will trip the breaker provided all is wired properly. It should trip the breaker quickly. An overload would not draw as much current as a short or fault but should trip the breaker in x amount of time depending on the overload level and trip curve of the breaker.
As far as question two, I don't think ferroresonance would be an issue with switching one phase. If you were adding impedance to the system (transformers, caps) then it might be an issue. However, if these are load break switches, they should not affect the model used in the power system study, which should have included ferroresonance issues. Maybe I am way off on this one but I dont think so. I have seen many, many load break switches on power poles and none of them were three pole for service which only used one phase. Of course, these were on non-utility owned lines.

 

[alehman]

I'm not sure I understand your questions completely, but I'll give it a try.

Q1: I think you are asking "Will the breaker trip due to ground fault because the load is connected line-to-neutral?". Normally a 15 amp breaker would not have ground fault sensing. If it does, it should be equipped with a neutral sensor which cancels the neutral current seen by the ground fault element. it should not trip if it is installed correctly. Did I guess right?

Q2: In most regions, power distribution to developments is installed and maintained by the utility company, which is not normally subject to typical building and electrical codes. In some countries, utilities are required to follow the National Electrical Safety Code, which is written specifically for utilities. In general it would not be appropriate to apply electrical codes intended for buildings to a distribution system for a development. However, the local authority has final say.

For your transformer taps, 1-phase vs. 3-phase switches will not make any difference with respect to ferrorseonance if the transformers and cabling are 1-phase. I don't understand what is meant by "interlocking mechanism". More information is needed.

 

[Engcan]

Thanks Alehman and Buzzp.

Here is more on my question #1.

What I am trying ask is, if a 15 A breaker connected line to neutral and comes across a line to ground fault on a 240 V single phase supply. Will the ground fault draw enough current to trip the breaker? I am asking this question because when we do fault studies, we know the zero sequence reactance of components and we can calculate but nothing we know in case of a small electric house panel and aslos ground faults are unpopular for drawing less current so they are difficult to detect. So my question stands will the breaker trip. Another thing, two statements of Alehman and Buzzp dont match:

Buzzp"A 15A breaker whos hot line contacts the ground, will trip the breaker provided all is wired properly. It should trip the breaker quickly"

ALEHMAN:"Normally a 15 amp breaker would not have ground fault sensing. If it does, it should be equipped with a neutral sensor which cancels the neutral current seen by the ground fault element. it should not trip if it is installed correctly. "

How these two statements can be justified when they are opposite to each other. I know in case of GFCI, it will trip but you will agree we dont use GFCI breakers in all the panels unless dictated by code. So there must be some logic for using simple NON-GFCI breakers that they will take care, if a ground fault happens.

Regards,

 

[bradh01]

Engcan,

Your 15 amp breaker should trip on a ground fault, assuming the wiring in your 120V single phase system is done properly. By this, I mean if the secondary of the 240/120V system is grounded, and the continuity of the grounding conductor system is maintained to the point of the ground fault. This ensures that the ground fault return path to the secondary of the 240/120V transformer is very low, and sufficient ground fault current will flow to trip the breaker. This concept of ensuring a "solid" grounding system is why the national electric code (NEC) makes such a big deal of maintaining the continuity of the grounding system (ground fault return path).

Also note that a typical home branch circuit breaker as you have here does not distinguish between ground fault, phase-to-phase fault, or load current. It simply trips at a pre-determined current level based on its tripping characteristic curve. In contrast, an industrial three phase power circuit breaker will likely distinguish between phase current and ground current (via current transformers and relaying) and may have seperate tripping characteristics for each.

 

[Engcan]

Hi Alehman,
More on interlocking mechanism, This is what ESA wrote:
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" Note: Rule 36-204 and Bulletin 36-17-0 require customer owned overcurrent protection ahead of all privately owned station equipment."
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Engcan: This simply means I beleive when the owners property will start they need to have a fused air ganag switch which will be coordinated with the utility recloser.

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Note: Rules 36-208 & 36-110 requires enclosure doors for all pad mount transformers to be interlocked with a ground operated group load break switch so that, access cannot be had to the transformer unless the load break switch ahead of the transformer is in the de-energized position, and the switch cannot be placed in the closed position until the transformer compartment has been closed.
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Engcan: The above mentioned block says about the interlocking mechanism, I plan to speak to S&C Electric about it but not sure they can provide with this interlocking mechanism. I am sure they make load break switches or this will be custom made.

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Note: Lightning arresters are required to be installed at both ends of the primary cables connected directly to overhead lines. Bulletin 36-8-4.
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Engcan: This last note seems weird to me actually, as far as my knowledge is concerned, a lightning arrester is concerned it is on the tap point of Overhead distribution line which is connected through a cable riser to the pad mounted distribution transformer.Normally an arrester called an Open point arrester is used on pad mounted transformer for 15 KV lines but in our case it is 4.8/8.3 KV. but there are other 6 pad mounted transformers connected to the one connected to the line and these are looped, so that might be the concern that ESA is asking for a primary bushing lighting arrester as well as overhead arrester.

I know I asked a lot of stuff here but for me to find from scracth might involve lot of hassle but for experts like you might involve some moments of your time. I would be grateful for these replies.
Regards,

 

[buzzp]

I think Brad clarified my statement more; the breaker will trip if the hot wire going through the breaker is the not wire which makes contact with a ground or neutral conductor. This is because all the current is going through the hot wire which goes through the breaker. I am fairly certain you do not have GF protection with your breaker.

Alehman, I believe, was saying the GF element would not trip in this case, if the breaker had this protection. But that depends on where the wiring is routed. I would have to believe if the GF protection was part of the breaker that the GF would trip if the fault current did not trip the breaker first since the return current (neutral) does not match the current going through the breaker (hot lead) since it is shunted at the location the hot wire touches the ground path.

Hopefully this clarifies my commmets further.

 

[alehman]

Q1:
I think we have a difference of semantics but are saying the same thing.

"What I am trying ask is, if a 15 A breaker connected line to neutral and comes across a line to ground fault on a 240 V single phase supply. Will the ground fault draw enough current to trip the breaker?"

It is installed properly and the source is solidly grounded, yes. It should trip whether it has ground fault sensing or not.

What I was trying say before is that if the breaker is equipped with ground fault sensing which is properly installed (meaning there is a properly wired neutral sensor), it will not see the neutral load current as a ground fault and therefore would not trip due to normal line-to-neutral load. Sorry if that was/is confusing.


Q2: The interlock requirement is something I have not seen before on pad mount switchgear or transformers. That sounds like a IEC requirement. Do they require a grounding switch as well? I don't think S&C has that as an option. You might also check with one of the custom shops such as Scott if no luck with S&C.

It is good practice to provide arrestors at transformers and both ends of cables. I have never seen that as a code requirement in the U.S.

 

[waross]

re the breaker. In North America the breaker will trip.

As bradh01 posted.

Also note that a typical home branch circuit breaker as you have here does not distinguish between ground fault, phase-to-phase fault, or load current. It simply trips at a pre-determined current level based on its tripping characteristic curve.

The system has to be solidly grounded. The neutral has to be connected to the ground, and and metal that is inadvertantly contacted by the hot lead must be grounded.

Any ground fault should draw sufficient current to trip the breaker, however, if grounding is not done properly, and the ground impedance limits the ground current to less than 15 amps it won't trip. It will depend on the design and workmanship of the installation.
Put another way, if the breaker doesn't trip on a ground fault your grounding is faulty.