SDS History

[bennie]

Article 2514, Alternating-current systems without Exterior connections, now known as Separately Derived Systems. Article 2514 was re-numbered in 1956.

This topic has always been in the Grounding Article, not in the transformer or service articles.

When this section was created, the separately derived systems,(premises wiring) were existing farm houses with generator power.

Farms with 32 volt light plants, did not require grounding even if they were separately derived systems.

The code section was created for the safety of farm houses when domestic power was installed.
Most existing separately derived systems did not have an earth connection.

Now with transformers being called separately derived systems, and being connected to earth in a second location, there is a lot of common mode current with resultant EMF.

Transformers are not separately derived systems and only need the equipment ground conductor for fault clearing.

Earth connecting the X-O of a transformer in two locations creates a ground loop, and is a violation.

 

[bphgravity]

Re: SDS History

Thanks for the info. I love electrical history and code origin!

 

[hornetd]

Re: SDS History

Bennie
I could really make good use of a truly isolated system that I could supply from the secondary windings of a transformer without a metallic pathway back to the utility's multi grounded neutral. My problem is that I have not discovered a way to build one that is in compliance with the NEC as written. In one case I was able to order 240 volt ungrounded service because the tariff for that utility did not limit 240 volt services to three phase only. I used the 240 to supply the primary of a separately derived system. Since niether of the supply conductors was grounded there was no pathway back to the utility MGN. Trouble is most utilities will not supply 240 volt ungrounded power in single phase.
--
Tom

 

[bennie]

Re: SDS History

Tom: I agree about breaking the connection to the utility MGN system.

The delta wye distribution system, with all star points common, provide protection for the utility system, from co-mingling voltages.

An isolation transformer will have the same voltage on both primary and secondary. A through fault will not impress high voltage on the secondary.

Up until 1984, the handbook schematics, showed a service with an ungrounded primary, and titled it a separately derived system. This was truly a component of a separately derived system.

Later on, the authors changed the drawings by adding in the ground system. This change eliminated the system from even being a component of a separately derived system.

The words "Separately derived system" should be deleted from the text of the code.

[ March 31, 2003, 03:44 AM: Message edited by: bennie ]

 

[bennie]

Re: SDS History

The UK uses 230/230 volt isolation transformers with an ungrounded secondary for bathroom power.

A motor/generator also provides electrical isolation.

[ March 31, 2003, 03:52 AM: Message edited by: bennie ]

 

[hornetd]

Re: SDS History

Originally posted by bennie:
The UK uses 230/230 volt isolation transformers with an ungrounded secondary for bathroom power.

A motor/generator also provides electrical isolation.

Actually It doesn't. I am required to ground the motor of the motor/alternator to the X0 of the supply transformer. So unless I can prevent any metallic pathway from existing to the alternator I will have a sneak current path.
--
Tom

 

[hurk27]

Re: SDS History

Bennie I think why the utilitys started requiring that the secondarys be grounded is the danger of a primary hot being knocked down and comming into contact with the secondary neutral. we had a truck that backed into a pole guy wire and snpaed the neutral going to a house and when it fell it hit a primary hot conductor and 7200 volts went into the house and started sevral fires at one time. but the fire department was just down the road and they saved the house. and the house also didnt have a water ground or a ground rod.

 

[bennie]

Re: SDS History

Hurk: I wonder how often the incident, you describe, has happened and written off as a breakdown in the premises wiring?

As I have posted before, I have seen the results of a 230 KV hit to a service, through the grounding system.

Everyone was going to call the damage equipment failure, until I showed them the hole blown in a furnace flue pipe that was hit by the line.

[ April 01, 2003, 05:04 AM: Message edited by: bennie ]