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Thread: No primary protection required?

  1. #41
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    Quote Originally Posted by Isaiah View Post
    Since its a separately derived system, a ground fault on the secondary of the first transformer will circulate indefinitely via grounded conductor from step up to step down xfmrs
    Still not following you here. The fact is, an ocpd on the primary of a 2 wire transformer will protect the second transformer. I agree with kwired it is protected per 450.3. The code is permissive. If they want to micromanage that protection then they need to tell us. Yes they do do that in other sections. Such as the transformer conductors, but IMO that should be taken out and rewrithen in an informational note.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Still not following you here. The fact is, an ocpd on the primary of a 2 wire transformer will protect the second transformer. I agree with kwired it is protected per 450.3. The code is permissive. If they want to micromanage that protection then they need to tell us. Yes they do do that in other sections. Such as the transformer conductors, but IMO that should be taken out and rewrithen in an informational note.

    Its not a two-wire system: Its a three wire secondary. You cannot protect both with a single OCPD on the primary.

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    saw in the graphics over the last week the same described that the ocpd on primary did protect the secondary. so confusing.
    Student of electrical codes. Please Take others advice first.

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    Quote Originally Posted by Adamjamma View Post
    saw in the graphics over the last week the same described that the ocpd on primary did protect the secondary. so confusing.
    Yes; but, section 450.14 requires a disconnecting means for the second transformer anyway, so why not make it a fused disconnect and remove all doubt?

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    Quote Originally Posted by Isaiah View Post
    Since its a separately derived system, a ground fault on the secondary of the first transformer will circulate indefinitely via grounded conductor from step up to step down xfmrs - it will not increase proportionally through the windings (1st xfmr primary side) until it goes phase to phase - then, the primary OCPD of will trip. Section 240.21(C)(1) explains when a single primary OCPD can be used to protect both primary and secondary conductors.
    This link from ECM magazine shows how a typical fault will flow on 3-wire secondary with grounded center tap:

    https://www.ecmweb.com/code-basics/g...erived-systems
    You have pretty much the same thing with what we are talking about. A fault to ground on secondary side of the first transformer, simply gives the secondary a ground reference and not much else happens. If you intentionally ground one of the secondary leads, then fault the other lead to anything grounded - this gives you a short circuit on that secondary, current will rapidly rise to levels that may be damaging to components. That current doesn't magically appear, it is powered by the primary coil, so current in the primary will raise proportionally to the current in the secondary. If secondary current increases by a factor of 5 then primary current increases by a factor of 5.Properly selected overcurrent device in the primary will protect both. When you have a multiwire secondary you need additional secondary protection because it is possible to overload a portion of the secondary but not the primary. Example 1kva transformer with 120/240 secondary, each half of the secondary is only rated for .5 kVA. If you put a .75kVA load on one half - that half is overloaded, but primary still sees less than it's 1 kVA rating so a primary only protection device would never catch this condition.

    Quote Originally Posted by Isaiah View Post
    Its not a two-wire system: Its a three wire secondary. You cannot protect both with a single OCPD on the primary.
    This discussion is about a two wire system. If one had a three wire secondary then you are correct.

    Quote Originally Posted by Isaiah View Post
    Yes; but, section 450.14 requires a disconnecting means for the second transformer anyway, so why not make it a fused disconnect and remove all doubt?
    If two wire system throughout the setup, I don't see why disconnect for first transformer can't also be the disconnect for the second transformer. If you are feeding a second building with second transformer at second building you have to comply with art 225 and should need a disconnect but it is because of the separate building more so than because there is a transformer there.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by kwired View Post
    You have pretty much the same thing with what we are talking about. A fault to ground on secondary side of the first transformer, simply gives the secondary a ground reference and not much else happens. If you intentionally ground one of the secondary leads, then fault the other lead to anything grounded - this gives you a short circuit on that secondary, current will rapidly rise to levels that may be damaging to components. That current doesn't magically appear, it is powered by the primary coil, so current in the primary will raise proportionally to the current in the secondary. If secondary current increases by a factor of 5 then primary current increases by a factor of 5.Properly selected overcurrent device in the primary will protect both. When you have a multiwire secondary you need additional secondary protection because it is possible to overload a portion of the secondary but not the primary. Example 1kva transformer with 120/240 secondary, each half of the secondary is only rated for .5 kVA. If you put a .75kVA load on one half - that half is overloaded, but primary still sees less than it's 1 kVA rating so a primary only protection device would never catch this condition.

    This discussion is about a two wire system. If one had a three wire secondary then you are correct.

    If two wire system throughout the setup, I don't see why disconnect for first transformer can't also be the disconnect for the second transformer. If you are feeding a second building with second transformer at second building you have to comply with art 225 and should need a disconnect but it is because of the separate building more so than because there is a transformer there.

    I wouldn’t feel comfortable with a fault travelling nearly two thousand feet, through secondary/primary windings and finally to the primary OCPD – seems like a lot of impedance. The disconnecting means for the second tranny primary is necessary from a maintenance perspective.

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    Quote Originally Posted by Isaiah View Post
    I wouldn’t feel comfortable with a fault travelling nearly two thousand feet, through secondary/primary windings and finally to the primary OCPD – seems like a lot of impedance. The disconnecting means for the second tranny primary is necessary from a maintenance perspective.
    Two thousand feet yes you might want disconnect even if not required.

    Was that kind of distance mentioned in OP? My replies have been general in nature.

    How much impedance over that distance depends on size and type of conductors, but one must also consider the OCPD setting. If you have a fault near the far end of the circuit, it may not draw 20kA, and get into instantaneous trip curve, but if you have a low level OCPD setting and still get a few hundred amps of fault current, it should trip just might take a little bit longer.
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    I have done this using a Sq D Mini Power zone. Primary, transformer, secondary panel, all in one box.
    Wire it, hang it, install two ground rods and you are done!
    Moderator-Washington State
    Ancora Imparo

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    Quote Originally Posted by kwired View Post
    Two thousand feet yes you might want disconnect even if not required.

    Was that kind of distance mentioned in OP? My replies have been general in nature.

    How much impedance over that distance depends on size and type of conductors, but one must also consider the OCPD setting. If you have a fault near the far end of the circuit, it may not draw 20kA, and get into instantaneous trip curve, but if you have a low level OCPD setting and still get a few hundred amps of fault current, it should trip just might take a little bit longer.
    Yes, the distance was mentioned in my initial post along with wire size. Youve made some good points but I prefer to err on the conservative side and add disconnecting means / primary overcurrent protection for the second transformer. We're not looking at huge cost since the loads are so small.

    Sent from my LM-X212(G) using Tapatalk

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    Quote Originally Posted by Isaiah View Post
    Yes, the distance was mentioned in my initial post along with wire size. Youve made some good points but I prefer to err on the conservative side and add disconnecting means / primary overcurrent protection for the second transformer. We're not looking at huge cost since the loads are so small.

    Sent from my LM-X212(G) using Tapatalk
    I just back to this thread.

    I disagree with some who have said that the primary OCPD of first tranny can protect the second tranny. Every tranny needs a disco and a primary OCPD.

    A primary tranny OCPD can protect the secondary depending on how you use 450.3.
    Under certain conditions this primary OCPD can also protect the secondary conductors, but that is it.
    "Electricity is really just organized lightning." George Carlin


    Derek

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