Sorry for the length of this, but it is a complex situation and I want to present as many of the pertinent details as possible.
I am working on a system to provide both legally required backup power to an existing hospital in addition to providing a backup for the full 3000A 277/480V/3 electric service. I am trying to cover three different scenarios.
First, the system will provide the standard arrangement for emergency power in a hospital through the the use of three automatic transfer switches (life ssafety, critical, and equipment). When a feeder for one of those branches or the utility source fails, the transfer switch(es) will start the generator and make the transfer within the required 10 seconds. These will be fed from an 800A load c/b on the generator.
Second, a 3000A manual transfer switch will be provided to select between the normal service entrance from the utility and the generator. This switch will be fed from a full load c/b on the 1500kw generator. If there is a pending storm, the owner can go to this switch, start the generator and make a seamless transition to emergency power (the switch is closed transition). This can also be done after a utility failure if the facility will be on the generator for an extended period of time and they wish to maintain full function.
Third, if the utility is experiencing heavy load and they call on the hospital to start the generator for interruptability, the same process as the second scenario would come into play.
Doing this within the constraints of an existing facility presents some interesting challenges. The point I would like to consider for discussion is the grounding of the system. The existing service consists of the pad mounted transformer and the service gear. There is a ground connection at the existing transformer and at the service disconnect. In order to make room for a building addition to house the generator and manual transfer switch, the utility transformer will be relocated. When this is done, the utility transformer will again be grounded in it's new location. I would then like to take advantage of the existing transformer pad and service entrance conductors to make connection to the existing service gear. When the manual transfer switch is added, it will have a service entrance rated breaker which will become the main service disconnecting means. Therefore, by code, a system of grounding electrodes and electrode conductors will need to be established at the transfer switch, and the ground and neutral will be bonded together at the switch location.
My dilemma is this: The existing eleven parallel sets of 500kcm service entrance conductors are installed in a concrete encased ductbact, but no ground was installed since these were between the utility transformer and the the main service disconnecting means. When the manual switch is installed, these conductors become a feeder and are required by code to carry a grounding conductor with them to the existing service gear.
For constructability, I want to minimize the downtime on the existing service, and reuse the existing copper if possible. Pulling a 400kcm grounding conductor into each of the eleven 4" PVC conduits does not seem practical without removal and reinstallation. There is a spare 4" duct in the duct bank, but code does not permit the grounding conductor to be installed in a separate duct from the paralleled sets.
Since the new manual switch/service disconnect and the existing service gear are only about 30 feet apart, would it be feasible to establish the new gounding electrode system and interconnect it with the one at the existing service and not put a grounding conductor in the feeder conduits?
I am working on a system to provide both legally required backup power to an existing hospital in addition to providing a backup for the full 3000A 277/480V/3 electric service. I am trying to cover three different scenarios.
First, the system will provide the standard arrangement for emergency power in a hospital through the the use of three automatic transfer switches (life ssafety, critical, and equipment). When a feeder for one of those branches or the utility source fails, the transfer switch(es) will start the generator and make the transfer within the required 10 seconds. These will be fed from an 800A load c/b on the generator.
Second, a 3000A manual transfer switch will be provided to select between the normal service entrance from the utility and the generator. This switch will be fed from a full load c/b on the 1500kw generator. If there is a pending storm, the owner can go to this switch, start the generator and make a seamless transition to emergency power (the switch is closed transition). This can also be done after a utility failure if the facility will be on the generator for an extended period of time and they wish to maintain full function.
Third, if the utility is experiencing heavy load and they call on the hospital to start the generator for interruptability, the same process as the second scenario would come into play.
Doing this within the constraints of an existing facility presents some interesting challenges. The point I would like to consider for discussion is the grounding of the system. The existing service consists of the pad mounted transformer and the service gear. There is a ground connection at the existing transformer and at the service disconnect. In order to make room for a building addition to house the generator and manual transfer switch, the utility transformer will be relocated. When this is done, the utility transformer will again be grounded in it's new location. I would then like to take advantage of the existing transformer pad and service entrance conductors to make connection to the existing service gear. When the manual transfer switch is added, it will have a service entrance rated breaker which will become the main service disconnecting means. Therefore, by code, a system of grounding electrodes and electrode conductors will need to be established at the transfer switch, and the ground and neutral will be bonded together at the switch location.
My dilemma is this: The existing eleven parallel sets of 500kcm service entrance conductors are installed in a concrete encased ductbact, but no ground was installed since these were between the utility transformer and the the main service disconnecting means. When the manual switch is installed, these conductors become a feeder and are required by code to carry a grounding conductor with them to the existing service gear.
For constructability, I want to minimize the downtime on the existing service, and reuse the existing copper if possible. Pulling a 400kcm grounding conductor into each of the eleven 4" PVC conduits does not seem practical without removal and reinstallation. There is a spare 4" duct in the duct bank, but code does not permit the grounding conductor to be installed in a separate duct from the paralleled sets.
Since the new manual switch/service disconnect and the existing service gear are only about 30 feet apart, would it be feasible to establish the new gounding electrode system and interconnect it with the one at the existing service and not put a grounding conductor in the feeder conduits?
