Intent of NEC 2017 section 517.13

[hhsting]

What is the intent of NEC 2017 section 517.13 as in terms of what happened that made it put that section? Is it because patients don’t get shocked and EGC are double due to low resistance and faster breaker tripping or is it because one EGC serves as backup in case one is damaged or totally some other reason. Does anyone here knowledgeable know the history, what happened and thought process behind NEC 2017 section 517.13? I did appreciate it if you are willing to share.

 

[augie47]

According to an IAEI magazine Article:
The purpose of the grounding portions of Article 517 of the National Electrical Code (NEC) is to reduce low voltage potential differences between electrically conductive surfaces that may be reached by a patient or care provider in the patient vicinity. Two independent means of grounding are required for the patient care areas and are connected in parallel. This limits the voltage and, thus, the electrical currents that the patient is exposed to and will reduce or eliminate the electrical shock hazard.

 

[don_resqcapt19]

Here is part of an article titled "Wiring Methods for Patient Care Areas", written by Fred Hartwell in the April 1994 edition of EC&M magazine. He was an editor of the magazine then.

History and intent

Many, particularly-those with an incomplete understanding of the background behind the rule, still argue that wiring methods using multiple equipment grounding conductors should be permitted, since they provide redundancy. The rule originated in the 1987 NEC and was seen by many as an unfair restriction against nonmetallic wiring methods. Accordingly, a serious attempt was made to sidetrack the requirement at the NFPA Annual Meeting.

Those attempts were met by negative arguments from a speaker who had extensively researched hospital grounding performance, particularly in older hospitals that had metal raceways in questionable condition. He found that the mere contact between the conductive surfaces of the raceways and normal building elements resulted in multiple grounding return paths, through concrete reinforcing, plumbing, building steel, and other paths. He found that the total impedance to ground over the raceway often ran between 10 and 50 times lower than the copper grounding conductor; duplicating that copper conductor would only reduce the impedance by a factor of two.

The attempt was defeated, but questions as to the accuracy of that research persisted. Another attempt to derail the requirement was made on the floor of the 1989 NFPA Annual Meeting. The same speaker responded, reporting on his more recent research. The research he had reported three years earlier was not correct, he said. Actually, the improvement in grounding return performance was between 20 and 100 times!

No serious attempt was made to rebut his arguments and the rule stayed in the 1990 NEC. No challenge was made at the Annual Meeting for the 1998 NEC.

The point is that the conductive outer wiring enclosure appears to significantly affect the equipment grounding performance of these systems, particularly in health care facilities where very small residual voltages on grounded equipment can cause severe problems.

It is for this reason that redundancy alone is insufficient; the outer enclosure itself must perform as an equipment grounding return path so the system will benefit from the other grounding paths. This is also noted in the fine print note that follows the allowance for isolated grounding receptacles in Sec. 517-16:

{FPN): Caution is important in specifying such a system with receptacles having isolated grounding terminals, since the grounding impedance is controlled only by the grounding conductors and does not benefit functionally from any parallel
grounding paths.

This editor attended the meetings reported here, and this editor wrote the version of Sec, 517-13(b) that now appears in the NEC. At some time, credible evidence may well emerge challenging the assumptions behind the rule. Until that happens, and the NEC is changed accordingly, wiring methods that do not use their outer enclosures as a qualified grounding return path are neither permitted nor intended.

 

[mbrooke]

IEC-60364-4-41 limits the conventional touch voltage to 25 volts during a fault, in addition to requiring a faster breaker opening time:




With breaker opening time not to exceed 0.3 seconds:



So how or why does this force redundant EGCs? Simple: ohms law


According to chapter 9 table 9, #12 conductor is 2 ohms per 1000 feet.

So, with a 200 foot run from a panel to a patient bed, the hot would have 0.4 ohms on the hot and 0.4 ohms on the EGC.

This would cause 150 amp of current to flow, opening a breaker in about 1 second worse case. So this governs clearing time.

Second, when a fault occurs a resistive divider is formed- 50% voltage drops across the hot 50% voltage across the EGC- so 60 volts to remote earth for up to 1 second.

Normally this would not be an issue according to the IEC body graph assuming 1000 ohms of body resistance.

However, in medical locations this values changes down to 400 ohms or lower.

Thus, a greater degree of protection is required.

So- now the same scenario with a metal raceway.

0.4 ohms on the hot

0.4 ohms on the combined copper EGC and 0.2 ohms on the conduit EGC whereby both create a parallel path down to 0.13 ohms.

The voltage division now changes whereby only 29 volts is present to remote earth.

Second, with the total ground fault loop impedance down to 0.53 ohms, the fault current increases now to 226 amps, making it likely the breaker will trip on its magnetic pickup (20x10=200).