Separately derived systems

[mooreaaryan]

I am installing a new service for a customer that need to be put on the grid. He currently has solar system feeding battery back-up. He will not be “back feeding” the grid with power. He wants a Manual transfer switch so that during the summer he can elect to be on grid while he charges his system and off grid to cool his house. In this scenario I believe it would be a separately derived system and that would force me to ensure that the grounded conductors of utility and solar do not make contact. Do I understand this requirement or am I missing something. Thank you

 

[Dsg319]

I am installing a new service for a customer that need to be put on the grid. He currently has solar system feeding battery back-up. He will not be “back feeding” the grid with power. He wants a Manual transfer switch so that during the summer he can elect to be on grid while he charges his system and off grid to cool his house. In this scenario I believe it would be a separately derived system and that would force me to ensure that the grounded conductors of utility and solar do not make contact. Do I understand this requirement or am I missing something. Thank you

I would imagine that would depend on wether or not your solar inverter has a floating neutral or bonded neutral.

I know nothing about solar and inverters. But coming from a generator aspect I would imagine that would be the same.

 

[winnie]

I think you may be missing the reason for 'SDS' rules. The basic requirements are all related to proper use of grounded electrical systems.

A grounded electrical system must have one supply terminal connected to ground and bonded to non current carrying materials at one (but only one) point. This, for example might be the bonding jumper location in a service or after a transformer.

When you have a generator with a transfer switch, the neutral to ground bond might be located at the generator, or at the service. If the bond is at the generator, then you must switch the neutral, otherwise your system neutral would be bonded to ground at two locations. But if the generator has a floating neutral then you must not switch the neutral, otherwise you would have an ungrounded system when operating on generator power.

In all cases, the neutral of the SDS ends up connected to the service neutral, via the collection of grounded metal. But there is one and only one such connection per system.

The question for you is 'how is this solar/battery/inverter setup electrically connected to the grounding and bonding system. Once you've figured this out you can decide the configuration of the transfer switch.

Jon

 

[jaggedben]

If it's an SDS or not in this case means two things:
- does the transfer-switch switch the neutral or just the ungrounded conductors
- is there a neutral to ground bond at both the service and the solar source, or only at the service.

You will need to look at how the solar system is currently configured. You may or may not have a choice depending on the equipment that is there.

For example, if the solar setup has a neutral-ground bond at the inverter that is not supposed to be removed, you'll need to make it an SDS. But if the N-G bond is only at the panelboard, you'll likely need to remove that bond and use a transfer switch that doesn't switch the neutral. Those are far from the only possibilities. If the solar equipment already has some type of micro-grid interconnect device (MID) that isn't being used, probably best to start using it, but those are rarely designed like an SDS and you should lift the N-G bond that is (supposed to be) existing. Solar equipment varies in these respects in a couple more ways than generators do.

 

[PaulWDent]

I am installing a new service for a customer that need to be put on the grid. He currently has solar system feeding battery back-up. He will not be “back feeding” the grid with power. He wants a Manual transfer switch so that during the summer he can elect to be on grid while he charges his system and off grid to cool his house. In this scenario I believe it would be a separately derived system and that would force me to ensure that the grounded conductors of utility and solar do not make contact. Do I understand this requirement or am I missing something. Thank you

No that would be dead wrong.
A solar or generator back-up system is a non-separately-derived system. The neutral of such a back up system must not be grounded in the back up system but should be connected to the grid neutral in the backed-up panel. The transfer swith should switch only L1 and L2 and neutral should never need to be switched.
By contrast, a separately derived system is one whose neutral is independently grounded already and you can't do anything about it. In that case, you must not connect the neutral of the separately derived system to the neutral of the main supply, as that neutral would then be grounded in 2 places, which is a no-no. In that case, the transfer switch must switch the load neutral from main to alternate supply SIMULTANEOUSLY with switching line(s).
NEVER do that for residential! Why? Legacy 3-pin tumble driers!

 

[PaulWDent]

On further reading of your post I understand that the existing solar system is the only system and thus has its neutral grounded already. You should remove that ground from the solar system's neutral and instead connect the solar system neutral to the neutral of the grid within the panel that the transfer switch feeds.

 

[jaggedben]

On further reading of your post I understand that the existing solar system is the only system and thus has its neutral grounded already. You should remove that ground from the solar system's neutral and instead connect the solar system neutral to the neutral of the grid within the panel that the transfer switch feeds.

While think that is preferred, if the existing solar equipment does not have a provision for unbonding the neutral then he is allowed to switch the neutral and do it as an SDS.

 

[Strathead]

No that would be dead wrong.
A solar or generator back-up system is a non-separately-derived system. The neutral of such a back up system must not be grounded in the back up system but should be connected to the grid neutral in the backed-up panel. The transfer swith should switch only L1 and L2 and neutral should never need to be switched.
By contrast, a separately derived system is one whose neutral is independently grounded already and you can't do anything about it. In that case, you must not connect the neutral of the separately derived system to the neutral of the main supply, as that neutral would then be grounded in 2 places, which is a no-no. In that case, the transfer switch must switch the load neutral from main to alternate supply SIMULTANEOUSLY with switching line(s).
NEVER do that for residential! Why? Legacy 3-pin tumble driers!

You write with such absolution. I know very little about solar in its entirety, but assume after it hits the inverters it is merely another AC source. Generators on the other hand, I am familiar with. While there would be no apparent reason to create a separately derived system and the cost is significantly more, there is nothing in the code that says you can't do that, so it would seem you are the one who is dead wrong. Review 250 before you make such assured statements. The neutral of the back up system can absolutely be bonded to the ground with a grounding electrode system. Then hots neutral and ground go to the transfer switch, which switches the neutral and the hots. The service does the same. No reason to do this, but no code reason not to.

 

[jaggedben]

You write with such absolution. I know very little about solar in its entirety, but assume after it hits the inverters it is merely another AC source. Generators on the other hand, I am familiar with. While there would be no apparent reason to create a separately derived system and the cost is significantly more, there is nothing in the code that says you can't do that, so it would seem you are the one who is dead wrong. Review 250 before you make such assured statements. The neutral of the back up system can absolutely be bonded to the ground with a grounding electrode system. Then hots neutral and ground go to the transfer switch, which switches the neutral and the hots. The service does the same. No reason to do this, but no code reason not to.

Small correction...Bonded with a system bonding jumper. But otherwise I completely agree.

 

[Strathead]

Small correction...Bonded with a system bonding jumper. But otherwise I completely agree.

Yeah, I should be more precise. I teach 2nd year APP. and tell my students precision is important.

 

[asif.k]

Your question is a bit incomplete. Apparemtly, it is an old Solar PV system wherein an old type of Inverter is installed. In that case, most probably, negative side of PV DC circuits is grounded and neutral of inverter output is also grounded. How the Grounding scheme is used is also important to know.
In case the scheme is as sated above, the DC side will continue to have the grounding scheme, while neutral ( inverter output) will also remain grounded however, incoming new service neutral needs to be switched or disconnected from the neutral of the Solar Inverter while the Solar system is charging the batteries.