I haven't seen any reply Iam wondering if the way as I wrote my questions are not clear enough.
Any feedback would be appreciated.
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We build electric furnaces.
I have been reading the Grounding Vs Bonding book along with NFPA 79, and I am more confused than before I started reading.
Our typical applications require to have one Main Disconnect Switch to feed several Zones of power .
The Customer provides a single input supply of 480V, 3 Phase, Ground Conductor.
Each Zone has it?s own Circuit Breaker , SCR and Power transformer.
The Main Disconnect switch is located in the same ?Power Enclosure? as the Circuit Breakers and SCR?s.
The transformers are located in the furnace Area which could be 20 to 30 meters away from the Power Enclosure.
The typical size of each transformer is 150KVA, The primary has a tap switch to change the secondary voltage between 20V to 80 V.
Question 1.
Should I consider this transformer as a System of Less than 50 V that should be grounded under 250.20 A(1) or Greater than 50V that shouldn?t be grounded under 250.21 A(1).?
Question 2.
Regardless if the transformer has to be grounded or not, Should I run a Grounding Equipment conductor from the Power Enclosure to each transformer, sized according 250.122 ?
The transformers are welded to the furnace frame, and the frame of the furnace is bonded to the grounding electrode conductor (Customer Scope).
Question 3
Let?s say one of the transformers has a secondary voltage of 60 V only, which under 250.21 A(1) doesn?t have to be grounded . How am I supposed to protect the transformer ?
Using the grounding equipment conductor from the Power enclosure feeding the transformer (Question 2) or could be considered already protected if the transformer is welded to the furnace frame ?
Question 4.
These transformers could drive more than 5000A in the secondary, If they have to be grounded under 250.20A(1), should I size the system jumper according 250.66 ?
Any help would be appreciated.
[ September 15, 2005, 10:08 PM: Message edited by: tester730 ]
Any feedback would be appreciated.
************************
We build electric furnaces.
I have been reading the Grounding Vs Bonding book along with NFPA 79, and I am more confused than before I started reading.
Our typical applications require to have one Main Disconnect Switch to feed several Zones of power .
The Customer provides a single input supply of 480V, 3 Phase, Ground Conductor.
Each Zone has it?s own Circuit Breaker , SCR and Power transformer.
The Main Disconnect switch is located in the same ?Power Enclosure? as the Circuit Breakers and SCR?s.
The transformers are located in the furnace Area which could be 20 to 30 meters away from the Power Enclosure.
The typical size of each transformer is 150KVA, The primary has a tap switch to change the secondary voltage between 20V to 80 V.
Question 1.
Should I consider this transformer as a System of Less than 50 V that should be grounded under 250.20 A(1) or Greater than 50V that shouldn?t be grounded under 250.21 A(1).?
Question 2.
Regardless if the transformer has to be grounded or not, Should I run a Grounding Equipment conductor from the Power Enclosure to each transformer, sized according 250.122 ?
The transformers are welded to the furnace frame, and the frame of the furnace is bonded to the grounding electrode conductor (Customer Scope).
Question 3
Let?s say one of the transformers has a secondary voltage of 60 V only, which under 250.21 A(1) doesn?t have to be grounded . How am I supposed to protect the transformer ?
Using the grounding equipment conductor from the Power enclosure feeding the transformer (Question 2) or could be considered already protected if the transformer is welded to the furnace frame ?
Question 4.
These transformers could drive more than 5000A in the secondary, If they have to be grounded under 250.20A(1), should I size the system jumper according 250.66 ?
Any help would be appreciated.
[ September 15, 2005, 10:08 PM: Message edited by: tester730 ]