6 ohms

[sparky59]

Are my calculations correct? A ground rod would have to have a resistance of 6 ohms to be able to clear a fault on a 120 volt circuit with a 20 amp breaker?

 

[dereckbc]

Re: 6 ohms

It shouldn't if the breaker is operating properly. You would only be drawing 20-amps, an acceptable (barely) load on a 20-amp breaker.

 

[haskindm]

Re: 6 ohms

Breakers are designed to carry 125% of their rated ampacity for 1500 seconds (30 minutes). In order to trip reliably and quickly in a fault situation a breaker should see 5 times its rated current. In order for a grounding electrode to provide an effective fault return path it would need to have a resistance much lower than 6-ohms. It would still be inviolation of that section that says the earth shall not be used as the fault return path.
Also remember that usually we are measuring resistance from the rod to the earth immediately surrounding the rod (using a 3-point, or fall-of-potential method) we really don't know what the total resistanc would be for current trying to return to "source" in order to trip a breaker.

[ December 15, 2005, 08:14 AM: Message edited by: haskindm ]

 

[don_resqcapt19]

Re: 6 ohms

haskindm,

Breakers are designed to carry 125% of their rated ampacity for 1500 seconds (30 minutes).

It appears from the following from a Square D document, that if the current is below 135% of the rating that the breaker may never open. The trip curves shown in the same document, also show that the breaker may never trip at currents up to 125-135% of the rating.

The tripping characteristics of molded case circuit breakers can be represented by a characteristic tripping curve that plots tripping time versus current level. The curve shows the amount of time required for a circuit breaker to trip at a given overcurrent level. Manufacturing tolerances result in a curve that is a band bound by minimum and maximum values of total clearing time. Total clearing time is the sum of the sensing time, unlatching time, mechanical operating time and arcing time of the circuit breaker. For currents in excess of 135% of the circuit breaker rating at rated ambient temperature (40?C), the circuit breaker will automatically open the circuit within limits specified by the band.

Don

 

[bphgravity]

Re: 6 ohms

I don't understand the intent of the question. Are you trying to prove that a ground rod will serve no purpose in clearing a fault therefore effective equipment bonding is needed, or are you trying to figure out a way to just use ground rods if you can lower the resistance enough?

 

[jim dungar]

Re: 6 ohms

To back up Don (not that he needs it though) and to prevent an arguement about breakers vs fuses.

Bussmann's FRN-R 15A 250V fuse time-current curve does not show the melting time for a 135% load either. Their curve stops at 300 seconds which is about 22A (147%).