In an earlier post, I asked for a circuit design that would keep the coax shield from becoming the neutral back to the transformer in the case of a broken power company neutral. There were some great insights into this issue posted, and I incorporated them all into the circuit design below. I would appreciate input.
The circuit you have will isolate your house from the utility grid. However, it may violate the National Electrical Code. My understanding (I am not an electrician) is that the first thing the power lines must encounter after the power meter is a main disconnect switch. This can either be stand-alone, or reside inside your primary electrical panel. That's fine, you can just add a switch to your schematic. However, the NEXT thing that must be encountered, right after your main disconnect switch is a bond between neutral and ground. Again, I am not an electrician, and you should consult one, or perhaps someone who has a copy of the NEC can look it up for you. So, from my understanding, you cannot put an isolation transformer that fails to ground neutral between your main disconnect switch and the bond to ground. In other words, there must be a connection between the neutral coming into your either your house, or into an outdoor electrical box, and your grounding system, i.e. the grounding rods and the cables that connect your grounding rods to neutral. In other, other words, you must allow the utility to send their neutral current through your ground rods.
The following topology might be allowed by NEC for isolation transformer. It seems to break the "rule" that neutral should be bonded to ground only after the main disconnect breaker, and nowhere else. However, I believe the presence of the transformer allows that "rule" to be broken. Check with an electrician who has installed such systems. Note, it may not be required to have a center tapped primary. If not, the connection from neutral to the primary center tap may be omitted. However, the bonding from neutral to ground after the main disconnect breaker I believe is required.
simulate this circuit – Schematic created using CircuitLab
If you do have a center-tapped primary on your isolation transformer, and you do connect that center tap to the utility's neutral wire, and bond it to ground, you will largely protect your house and belongings from a lost neutral. The primary of an isolation transformer will not care if the neutral is missing, and even if the secondary is unbalanced, resulting in current flowing through the secondary center tap, the primary will still be balanced. If the secondary is unbalanced, however, the transformer has to be rated to handle the maximum current that might flow through a single half of the secondary coil. You can't just take the amp rating of the main breaker switch, multiply it by voltage and use the resulting value for the volt-amps of the transformer. The transformer manufacturer may (and likely will) rate the transformer for the case where the load is evenly distributed between the two halves of the secondary coil.
An isolation transformer rated with enough volt-amps to supply a typical house will be heavy, bulky and expensive. However, as you have already suffered through two lost neutrals and subsequent damage to electrical devices, it may be worth it.
An alternative that is often used to provide protection against lost neutrals is to have a breaker that will trip on over-voltage. What kills your devices in the case of a lost neutral is that the voltage between one of the lines and ground decreases, while the voltage between the other line and ground increases. Shutting off the power when the voltage rises too high protects your devices.
37.5Kva transformer. Neither the power company nor the internet provider (coax installer) came up with a solution to protect the house from fire or equipment damage in the case of another neutral break, which is fully expected given the route through a forest of the two services. The internet provider said I can do whatever I want after they install service. NESC section 9 092 4. states interruption of continuity between ground connections is a solution to an objectionable flow of current due to presence of multi-grounds. This has been measured and is excessive during normal operations, and appropriate isolation measures taken as indicated in the circuit design. Three 8' ground rods installed, along with the equipment indicated in the final circuit design. Thanks for the input from this forum.
