I was wondering if anyone knows of a product (NEC legal) like a blocking diode or in-line fuse/breaker that can protect a house/service from getting accidentally powered through the service neutral. The reason i ask is because where i live and work, this has happened and houses have burned down. Most of the homes are unoccupied for 8 months of the year and the power is shut off. In these cases, the high voltage primary wire broke and came down to feed the secondary neutral which in turn fed all the homes connected to that neutral. Without any protection at all on the homes' neutral, ALL wires connected got fed and burned. Is there any way to protect a home from this?
Service neutral protection
- Thread starter dala
- Start date
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
It must have taken the primary protection a long time to clear the fault. A high/medium voltage ungrounded conductor coming into contact with a grounded conductor should have cleared an upstream utility fuse/breaker. Of course they are set pretty high, but maybe it is only something the utility would be able to protect. Any protective relays at the home would be referencing ground, which is connected to the neutral, so other than an initial transient, I'm not sure how you would sense it.
LISHAJI
Member
- Location
- Albany, NY
This would not necessarily cause an hazardous overvoltage, as long as the distribution transformer remains in the circuit. But, if the transformer fuse operates to separate the transformer from the circuit, before the upstream line fuse, the secondary voltage rises to the primary voltage. Normally this happens, when transformer is too tightly fused; smaller, fast transformer fuses are more likely to clear before an upstream device. or when the upstream device is not a fuse, rather a breaker or recloser with time delay trip! Take it up with the utility!!
The real problem is the utility service. In these cases, it took them at least 20 minutes to shut the power off. The high voltage primary had been bypassing the secondary and current was pushing to ground using the neutral in all houses connected and properly grounded. My goal is to create a procedure for a seasonal homeowner to be able to completely and safely disconnect their house from the grid during the time it is "Winterized". And turn it back on when they come back and start using it again for the season. Would disconnecting the ground wires from the ground rods and water pipes work??
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
I don't know if that is possible. Current finds all paths back to its source neutral, so even if you replaced all the two pole service mains in the homes with 3 pole and had a switched neutral at the panel, the ground would be energized relative to the floating or erratically energized neutral.
kwired
Electron manager
- Location
- NE Nebraska
I read this several times and am having a hard time visualizing what is happening.
Primary ungrounded conductor contacts secondary grounded conductor. This should interrupt primary overcurrent protection immediately. Is the primary and secondary neutrals not bonded together? If not the owners of these homes are lucky they were not there when it happened otherwise they are subject to electric shock at primary to ground voltage if they are touching anything bonded to the electrical grounding system.
LISHAJI
Member
- Location
- Albany, NY
I don't think neutral voltage can be any different than ground voltage in the house due to this fault, unless the wiring is not right. Neutral is theoretically at ground potential at the service disconnect if it is properly grounded. But if the structure is better grounded than the service ground then all the kind of problem you mentioned would rise. There would be a voltage between the neutral and structural ground which would appear on the metallic body of the appliances in contact with the structure, resulting in failure, specially of the electronics.
Way out is check the service ground and should be less than 10 ohms (NEC requirement is way too high). Also check ground bond to all structural items as mentioned in NEC. (water lines, sewer, foundation rebar if available etc.) Install a Surge protector type 1 at the service disconnect.
I don't know if the utility company has the primary and secondary neutrals bonded together. Maybe not??? Is it possible that there were enough paths for current to flow creating enough resistance to not trip an overcurrent device? All i can say is that, as a firefighter at the scene, when we got to the third house that was smoking and tore off the siding and sheathing from the outside wall where the circuit breaker panel was, every wire going up to it was glowing red. All wires stapled to the joists also were glowing and burning the joists. At this point the other 2 houses were burnt down. As an electrician, I'd like to also provide my customers with better answers as to why it happened. More so , i'd like to put something in place to prevent such a scenario and put them at ease. Do you know of any way (legally) to completely disconnect a house from the utility and turn it back on when wanted?
kwired
Electron manager
- Location
- NE Nebraska
I think the POCO has issues and doesn't want to make any public statements about them.
You need to look around at the primary system in the area. It is not hard to tell if the primary neutral and secondary neutral are bonded together - most of the time they are the same conductor. The bonded together grounded conductors of the two systems essentially becomes a common conductor to both systems, if the primary comes into contact with it there will be low enough impedance the primary overcurrent device will respond quickly.
If the primary system is a delta system there may not be a neutral, there should still be some kind of ground detection system. If there is no primary neutral each single phase transformer will have obvious connections to two phases. Delta systems are more common on transmission voltages AFAIK, and distribution systems is mostly wye systems.
T
T.M.Haja Sahib
Guest
If you use a 4 pole main circuit breaker at the service entrance for 3 phase service for example ,it would trip for high fault current initially flowing only in neutral,thereby protecting the wires inside the house.
LISHAJI
Member
- Location
- Albany, NY
How can this be done? Even if you break the neutral for a fault, it still would remain connect through the ground connection. It would be violation to have supply or load side neutral isolated from ground. Alternatively if you would run EGC from transformer to the service with isolated neutral from ground, it won't serve the purpose you mentioned!! It would still be the status quo!!
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That's interesting. If it's single phase i'd use a 3 pole breaker? Would 1 of the poles of that breaker get the neutral? Would i then use a splice through breaker style connection to feed the bus bar? If not, how would i make the connections? Is this totally NEC and AHJ acceptable?
T
T.M.Haja Sahib
Guest
Using 3 pole main breaker for single phase service and 4 pole main circuit breaker for 3 phase service is not a violation because neutral is cut off along with phases when the breaker trips.
If the wiring is properly done in a home,the fault current, due to contact of high voltage line with low voltage neutral,can not enter into the building wiring via the neutral wire because the neutral is grounded at the service entrance.But if the wiring is improper i.e if there is a ground fault of neutral wire inside the building,the breaker can help protect the wiring during such faults as a stop gap measure.
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- Location
- Massachusetts
No it is not.
- Location
- Massachusetts
You are forgetting the bonding which is a violation to break.
T
T.M.Haja Sahib
Guest
Break in EGC is not required ........
- Location
- Massachusetts
TM, do you have any experience at all with American services?
At the service the EGC and the Neutral are the same conductor.
T
T.M.Haja Sahib
Guest
Please put up a drawing to clarify the issue,if possible.
- Location
- Massachusetts
Enjoy
http://electrical-engineering-portal.com/neutral-single-earthed-or-multi-earthed
We use the multi-grounded neutral system here which means going into a building is a single conductor that serves as both the neutral and the grounding means.
T
T.M.Haja Sahib
Guest
Hmm..I wanted a diagram of service entrance equipment wiring layout......Something else was provided...
The neutral and EGC are distinct wires after the service.Just at the place where they emerge as distinct wires, placing the 4 pole breaker to cut off the neutral along with other phases during tripping would serve the purpose.
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