Corner grounding a delta-delta transformer

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2Broke2Sleep

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Can someone help me understand the code requirement of 240.22. I just can't understand what it is saying. If you have the mike holt dvd "Transformer Calculations" from 2014 its discussed at the 15:00 minute mark in the bonus material section "grounded and ungrounded systems"

240.22 Grounded Conductor
No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met:
1. The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.
2.Where required by 430.36 or 430.37 for motor overload protection.

They are talking about terminating the ungrounded conductor in a 3 phase disconnect on one of the lugs and installing a bar in between. Or a 3 pole CB where one of the phases will be white....
 
2Broke2Sleep said:
Can someone help me understand the code requirement of 240.22. I just can't understand what it is saying. If you have the mike holt dvd "Transformer Calculations" from 2014 its discussed at the 15:00 minute mark in the bonus material section "grounded and ungrounded systems"

240.22 Grounded Conductor
No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met:
1. The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.
2.Where required by 430.36 or 430.37 for motor overload protection.

They are talking about terminating the ungrounded conductor in a 3 phase disconnect on one of the lugs and installing a bar in between. Or a 3 pole CB where one of the phases will be white....
Click to expand...
You either need to use a two-pole circuit breaker on the two ungrounded lines and feed the neutral through by solid connection to the supply or use a common-trip 3-pole breaker.
 
From what I understood it was a 3-phase load. This is what lost me. It could just be Mike's ADHD changing the design of the system mid-sentence without informing anyone....
 
2Broke2Sleep said:
From what I understood it was a 3-phase load. This is what lost me. It could just be Mike's ADHD changing the design of the system mid-sentence without informing anyone....
Click to expand...
With corner-grounded delta you can use a two-pole breaker for 3-phase loads. How is the panelboard wired?
 
Smart $ said:
With corner-grounded delta you can use a two-pole breaker for 3-phase loads. How is the panelboard wired?
Click to expand...

I don't know, this was a hypothetical example. I don't have any experience with delta-delta so maybe I would understand better if I had encountered it before..
 
240.22 is talking about grounded conductors in general, not just for corner grounded systems.

Do you ever put overcurrent devices on the neutral of a 120 volt circuit unless the overcurrent device switches the ungrounded conductor simultaneously (think GFCI circuit breaker)?

It is saying to never put an overcurrent device on a grounded conductor unless all ungrounded conductors of the same circuit are opened simultaneously with the grounded conductor.
 
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2Broke2Sleep said:
I don't know, this was a hypothetical example. I don't have any experience with delta-delta so maybe I would understand better if I had encountered it before..
Click to expand...
I estimate a high probability you will never encounter a corner-grounded delta secondary... but we can discuss it just in case you do. :D

As with any isolation transformer secondary, you can ground one point of the system. We typically ground the wye point of a wye secondary or a center tap of one winding on a delta, but you could ground any other one point on the system instead and all you would do is change the voltage to ground of the other points.

You do not want to ground two points. :happyno:

With that said, you would treat the conductor(s) that run from that grounded point just as you would the 'neutral' of the conventional systems we run, i.e. on a wye or center tap delta high leg system. As such we typically just make solid connections to these grounded neutral conductors with no OCPD. An OCPD in the grounded conductor will not trip on a grounded conductor to ground fault. Logic precludes to install an OCPD in this conductor is pointless. Yes it would trip if there were a grounded conductor to line fault, but if we have an OCPD in the line conductor, the circuit is already protected and putting an OCPD in the grounded conductor is unnecessarily redundant.

So enter the corner grounded delta secondary. One line conductor is grounded (conventionally it is BØ). We can treat it just like a grounded neutral in our other conventional systems and make solid connections in distribution with no in-line OCPD. You can run a typical two-wire line-to-line load off just a single pole breaker because on this system it is technically a line-to-ground load. With a three phase three wire (3Ø 3W) load you just need a two-pole breaker... as the third wire is already grounded. The panel board will appear wired the same as a 120/240 1Ø 3W system.

Now, with all that said, some parties (I don't really have any names to mention :p) may prefer to run this system's secondary the same as a tradition 3Ø 3W delta, using two-pole breakers for line-to-line loads, and three-pole breakers for 3Ø 3W loads... using a standard 3Ø panelboard. In doing so, any pole connected to the grounded conductor must have a common trip with the ungrounded conductor poles of the same circuit just in case there is a nuisance trip on the grounded conductor. Using a three-pole fused disconnect switch you'd have to use a dummy fuse or bypass bus in the grounded conductor fuse slot as the lines are common switch, but not common trip.
 
2Broke2Sleep said:
They are talking about terminating the ungrounded conductor in a 3 phase disconnect on one of the lugs and installing a bar in between. Or a 3 pole CB where one of the phases will be white....
Click to expand...
The issue at play in this last sentence is that if you use a FUSED DISCONNECT, you cannot put a fuse in the grounded conductor, because that fuse could open and NOT disconnect power to the other two phases. So if it is a Fused Disconnect, the grounded leg gets a solid connector, no fuse (AKA "Dummy Fuse" or "Slug"). If you use a 3 phase CIRCUIT BREAKER, that's OK because when the breaker trips, is opens all three phases simultaneously.
 
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