SDS Transformers Source of Power

hitehm

Member
Location
Las Vegas NV
I know this is a pretty dumb question but every time I search on anything to do with Separately Derived Systems ALL I ever get is info on grounding and bonding them. What I want to know is: What exactly is the purpose when the system is a transformer? I understand that there are many times you want or even must use power not from the utility and these "off grid" systems are essential for those times as either main power or standby power. PV, generators, UPS, wind, inverters etc can all provide off grid power ON THEIR OWN when needed but doesn't a transformer get it's primary power from the utility Co? If so then, other than increasing or decreasing the voltage for a special purpose, what is the point as a SDS? If the utility "service" goes down the transformer goes down. So what am I missing or am I misunderstanding what a SDS is used for?
 

ron

Senior Member
The reason when you search SDS definitions are always tied to grounding and bonding connections is because that is how Code Making Panel 5 of the NFPA 70 defined it.
Separately Derived System. An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.
A "regular" delta - wye transformer is an SDS because there is no direct connection between the primary and secondary windings. That is not the case for an autotransformer for example.
It is not an SDS because it is or is not fed from a fuel cell or utility.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Another way to look at it: Everything up to and including the transformer primary is a load like any other, and requires equipment grounding tied to the source neutral, which assures that over-current protection functions. That reason the neutral is grounded is because the POCO transformer's secondary's neutral is grounded.

Your new, separately-derived secondary is a brand new source, just like the POCO transformer, and it won't contain a grounded conductor until you intentionally ground one. So, just like the POCO does, you must choose a conductor to ground, so your new over-current protection also functions if there's a ground fault.

In other words, there's really no functional difference between their transformers and yours; if you need a grounded conductor (often known as the neutral), you must derive one. There are no electrical connections between the primary and the secondary, until you tie your new neutral to theirs, since there is only one ground.

Once you've done that, your secondary is no longer an isolated, floating electrical system like an ungrounded delta is. There are really relatively few true separately-derived system in use, since tying the new neutral to the existing grounding system interconnects the two neutrals, but that somehow doesn't seem to count.
 
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I know this is a pretty dumb question but every time I search on anything to do with Separately Derived Systems ALL I ever get is info on grounding and bonding them. What I want to know is: What exactly is the purpose when the system is a transformer? I understand that there are many times you want or even must use power not from the utility and these "off grid" systems are essential for those times as either main power or standby power. PV, generators, UPS, wind, inverters etc can all provide off grid power ON THEIR OWN when needed but doesn't a transformer get it's primary power from the utility Co? If so then, other than increasing or decreasing the voltage for a special purpose, what is the point as a SDS? If the utility "service" goes down the transformer goes down. So what am I missing or am I misunderstanding what a SDS is used for?
Generally, one doesnt strive to make/use an SDS. Its generally not something you "need" in one situation or another. Rather, it is the result of using an isolating transformer to change voltage. When you do that, it has the unfortunate* effect of creating a new electrical system with no ground reference to the previous system. It is important to know when you have created an SDS, otherwise it could be a floating electrical system with no ground reference or fault clearing ability.

*there are some special situations where this is specifically a good thing. Example: Once I needed 240v with one conductor being a grounded conductor for a foreign machine. I was able to make that system with a transformer.
 

Carultch

Senior Member
Location
Massachusetts
but doesn't a transformer get it's primary power from the utility Co? If so then, other than increasing or decreasing the voltage for a special purpose, what is the point as a SDS? If the utility "service" goes down the transformer goes down. So what am I missing or am I misunderstanding what a SDS is used for?
A transformer gets its POWER from the utility, but that's really the essence of what it transmits The transformer sides are electrically isolated from one another. If you take a transformer out of the box, and attempt to Ohm-meter from the primary side to the secondary side, you'll get infinite ohms, and won't measure any continuity.

The transformer magnetically couples the two windings together, so that they transmit power through the magnetic field, rather than through a wired connection. The primary coil produces a magnetic field from its electric current. The secondary coil picks up the time-variance of that magnetic field and generates its voltage. The time-variance of the alternating current allows it to transmit power for AC, but not for DC, because Faraday's law of induction depends on a changing magnetic field. It is the simultaneous operation of Ampere's law and Faraday's law, that allow the transformer to work for re-formatting the mix of voltage and current in electrical power transmission, using a magnetic field as a medium. Just like a pair of gears re-formats the mix of torque and speed in mechanical power transmission.

The primary coil can only induce the changes in voltages onto the secondary coil, in proportion to its own changes in voltage. The DC offset of the sine wave doesn't get through, and the reference to ground doesn't get through. You have to re-ground the system derived from the transformer secondary, in order for it to be a grounded system. This is what is meant by "separately derived systems" in the NEC.
 
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jaggedben

Senior Member
I know this is a pretty dumb question but every time I search on anything to do with Separately Derived Systems ALL I ever get is info on grounding and bonding them. What I want to know is: What exactly is the purpose when the system is a transformer? I understand that there are many times you want or even must use power not from the utility and these "off grid" systems are essential for those times as either main power or standby power. PV, generators, UPS, wind, inverters etc can all provide off grid power ON THEIR OWN when needed but doesn't a transformer get it's primary power from the utility Co? If so then, other than increasing or decreasing the voltage for a special purpose, what is the point as a SDS? If the utility "service" goes down the transformer goes down. So what am I missing or am I misunderstanding what a SDS is used for?
Transformers are used to change voltage. That's what they are for.

The only thing that most transformers have in common with 'off-grid' systems is that, by creating a galvanically isolated system (i.e. no electrons flow directly through wires from one system to the other), each typically creates a need for a new grounding point for the isolated system. That is why you read so much about grounding and bonding SDSs in the code.

A transformer is typically powered by a utility, but say it were powered by an off-grid source such as a generator, you would still have separate systems on each side of the transformer, each typically needing to be grounded.
 
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