Joe, Yes, we have to bond around this, but what about the plumber who is replacing the broken metal underground water pipe out in the yard. The NEC requirement to use the metal underground water piping system as a grounding electrode creates a serious safety hazard for the plumbers. Why should the NEC create this hazard for others? Don(resqcapt19)
#103187 - 05/30/0211:40 AMRe: Is there a shock hazard here!
The hazard is aways there even without an open neutral problem. In a code compliant installation the metal under ground water pipe is in parallel with the grounded conductor. In some cases the water pipe has a lower impepedance then the service grounded conductor and carries more than half of the grounded conductor current. Don(resqcapt19)
[This message has been edited by resqcapt19 (edited 05-31-2002).]
#103190 - 06/01/0209:54 AMRe: Is there a shock hazard here!
Yes, it is if you take it apart while it is energized. The hazard I am talking about is when the unsuspecting plumber comes to work on the water pipe and takes it apart and gets between the two pieces of the pipe. This hazard exists under normal conditions in a code compliant installation where there is a metal underground water pipe system. Under ground fault conditions, while the fault exists, there is a shock hazard between all conductive things that are bonded to the electrical grounding system and the "earth". This voltage between the bonded items and earth will be equal to the voltage drop on the EGC that is carrying the fault current. This voltage can be in the 40 to 50 volt range and even higher if the EGC is not of the correct size or not properly sized. This voltage will exist until the fault is cleard which should be within a very short time is the OCPD is correctly sized and functioning. Don(resqcapt19)
[This message has been edited by resqcapt19 (edited 06-01-2002).]
#103191 - 06/01/0203:19 PMRe: Is there a shock hazard here!
I know of one installation where a 2W 1Ø 120V service to a dwelling, the Water Grounding Electrode Conductor (GEC) carried 2/3rds of the Grounded conductor current, ie. the grounded conductor between the Main Switch and the Utility Transformer across the street read about 8 Amps and the ungrounded Conductor read about 24 Amps and the Water GEC read about 16 Amps.
There was 8 Services off of the Transformer and this unit was the only 2W Service drop.
Even after a 3W service was installed, the water GEC still carried about 2/3 of the neutral current ( determined by turning "off" all of L1 then 'in turn' turning "off" all of L2 CB's) and reading the ungrounded conductor ampeage vs the grounded conductor amperage.
Do not know status of the other services on that transformer.
The City choose not to do a follow-up on why.
#103192 - 06/01/0206:45 PMRe: Is there a shock hazard here!
Al, The water pipe is always a hazard, not only when there is a fault, but always. It can even be a hazard when the main for the building is off. It is being used to carry part of the grounded conductor current and in some cases a majority of that current. Any time the pipe is "opened" and you touch both sides, you become part of the circuit. Look at what the code requires. It says we must bond the grounded conductor to the metal underground water piping system, and so does your neighbor. If you have a 100 amp service with a long service drop and next door has a larger service with a shorter service drop and both homes are on a city under ground water system, the path via the water pipe to next door and then to the transformer may have a much lower impedance then your service drop grounded conductor. The grounded conductor current will be split between these two paths. The amount on each path will be determined by the impedance of the two paths with the most current on the path with the least impedance. As far as the touch voltage at the time of a fault, it exists on everything that is bonded to the electrical system grounding system. The most touch potential will appear on the enclosure where the fault occurs, but the voltage drop on the grounded conductor between the main bonding jumper and the utility transformer will appear on everything that is connected to the electrical grounding system. This could still be a high enough voltage to cause serious harm. Fortunately this voltage to ground only exists as long as the fault exists. In a properly designed, installed and maintained system the fault should only exist for a very short time. Don
#103193 - 06/01/0207:12 PMRe: Is there a shock hazard here!
The water pipe jumper is an easy thing to identify as missing, the plumbers need to be even more careful where opening gas lines. We had an instance where a gas line was carrying 6 - 8 amps of neutral current. The gas technician shut off the up stream valve, and when he broke the connection, the arc ignited the residual natural gas. This burned the hair off both his arms, he was very lucky. we found the neutral connection at the weatherhead had failed & the gas pipe was acting as the house neutral. (The water dept had experimenting with PVC & this house had a pvc water pipe from the street to just outside the cellar.)There was no driven grounds as the service was 30 - 40 years old.