250.21(B), Ground detectors

[jim dungar]

The HO and XO are grounded, would you consider that a SDS?.

Separately derived systems are allowed, actually they are required, to have common grounding connections.

Most utilities 480Y/277V and 208Y/120V services are fed from Yg-Yg transformers.

 

[Deltaforce]

The HO and XO are grounded, would you consider that a SDS?.
They are all 70's era, I know the definition has changed slightly I am genuinely curious what your take is?

The code still consider to be sds as no direct connection between two neutrals
However the issue, code intent of sds to mitigate objectionable current in premise star-star connected transformer no met this way:
As both primary and secondary neutrals connect to building GES, any MV ground fault may cause part ground fault current flow through EGC to LV neutral, LV netral to GES. GES to primary neutral, primary neutral to MV utility source
As egc, lv neutral connections may not rated such mv fault current. they may damage

 

[jim dungar]

As both primary and secondary neutrals connect to building GES, any MV ground fault may cause part ground fault current flow through EGC to LV neutral, LV netral to GES. GES to primary neutral, primary neutral to MV utility source
As egc, lv neutral connections may not rated such mv fault current. they may damage

In the US, all neutrals, across the entire country are electrically bonded together. For utilities this may occur every couple of kilometers as part of their multi-grounded neutral MV distribution. For LV systems this occurs at each service or each transformer secondary. About 30 years ago, the NEC effectively discontinued the use of isolated systems, except in hospital ORs.

 

[Deltaforce]

In the US, all neutrals, across the entire country are electrically bonded together. For utilities this may occur every couple of kilometers as part of their multi-grounded neutral MV distribution. For LV systems this occurs at each service or each transformer secondary. About 30 years ago, the NEC effectively discontinued the use of isolated systems, except in hospital ORs.

Hrg system in primary neutral star-star transformer building premise effective solution, prenvent this objectionable current, perhaps compliance(?)250.6

 

[jim dungar]

Hrg system in primary neutral star-star transformer building premise effective solution, prenvent this objectionable current, perhaps compliance(?)250.6

Are you trying to say that an HRG installed on the secondary side of a service transformer might prevent some kind of objectionable currents someplace? This is not the primary reason I have installed them in the past.

The NEC does not allow the general use of impedance grounding. There are restrictions as to the electrical maintenance capability.

 

[Deltaforce]

Are you trying to say that an HRG installed on the secondary side of a service transformer might prevent some kind of objectionable currents someplace? This is not the primary reason I have installed them in the past.

The NEC does not allow the general use of impedance grounding. There are restrictions as to the electrical maintenance capability.

HRG not in secondary neutral, in primary neutral.

 

[jim dungar]

HRG not in secondary neutral, in primary neutral.

Utilities do not use HRG on their distribution network to customer services.

 

[Deltaforce]

Utilities do not use HRG on their distribution network to customer services.

Let say service point upstream transformer So star-star transformer owner's. Can owner put hrg primary neutral?

 

[jim dungar]

HRGs are allowed on customer owned services. Very few customers use Yg-Yg transformers, it is more typical to use Y-Yg or D-Yg.
Also MV systems may use reactors instead of resistors.


Edit: darn autocorrect.

 

[tortuga]

Hugs are allowed on customer owned services.

I had a customer give me a hug the other day after I got her power back on, I figured it was allowed on customer owned services.

 

[Deltaforce]

, it is more typical to use Y-Yg

Y-Yg use prevent mv ground fault current flow, zero Sequence current flow to utility mv source, though it introduce other issues, if without tertiary delta winding

 

[Jpflex]

No it is not.
You apparently have two "operating systems", one >1000V and another at 480V. I am pretty sure you have some more like 208V and maybe even some DC.

Each system stands on its own. See the NEC definition of separately derived systems.

So when nec mentions the system operating under 1000 volts, we assume they are only considering secondary output but not primary input voltage level?

 

[jim dungar]

So when nec mentions the system operating under 1000 volts, we assume they are only considering secondary output but not primary input voltage level?

It is over 1000V if you are talking about the primary side system components. It is not over 1000V if you are talking about the secondary side system components.
The transformer has components from both systems.

This is what allows us to use Low Voltage wiring for door bells and thermostats without worrying about the primary side of their transformers.