517.13 - Explanation

[Keri_WW]

In layman's terms can you explain the requirements and reasoning for NEC 517.13?

Thanks,
Keri

 

[hurk27]

ok in laymans terms, is simple.

While a doctor has you opened up and is working on you while you lay on an ice cold stainless steel table, the NEC doesn't want any chance of a difference of potential between that table and the tools or monitoring equipment that a doctor might be working on you with, which is at a time you are most subject to electrical shock, by redundant grounding paths all equipment can be kept at the same potential, or is supposed to.

 

[raider1]

Here it is in a nutshell.

The wiring methods used in patient care areas must itself qualify as an equipment grounding conductor in accordance with 250.118. This means that cable wiring methods must be AC type cable or MCAP. Non-metallic cables and standard MC cable are not permitted to be used in patient care areas. EMT, RMC, IMC are all permitted but PVC and other non-metallic raceways are not permitted.

In addition to the requirement that we use metal wiring methods the branch circuits in patient care areas must also contain an insulated copper equipment grounding conductor. The exceptions to 517.13(B) only permit the omission of the insulated EGC not the requirement for a metal wiring method that itself qualifies as an EGC.

The reason for the "Redundant grounding" in patient care areas is that the combination of a metal wiring method and an insulated copper EGC lowers the impeadance of the fault current path and helps to prevent a fault in a piece of medical equipment from passing through a patient and in addition it helps to limit the touch potential that can occur in grounded equipment due to the voltage drop in a single wire type EGC.

Hope this helps.

Chris

 

[roger]

Along with the other posts here is what the authors of NFPA 99 have to say about the reason for both EGC's


NFPA 99 A 4.3.3.1.3

The performance of the grounding system is made effective through the existence of the green grounding wire, the metal raceway, and all of the other building metal. Measurements have shown that it is the metal raceway and building steel that provide most of the effective grounding path of less than 10 milliohms at the receptacle, including plug to receptacle impedance. The green grounding wire becomes a backup, not a primary path performer.

The fact that a metal raceway (even emt) is a better EGC than a wire conductor is hard for some to swallow, but never the less, it is a fact that it is recognized as such.

Roger

 

[Keri_WW]

So as long as you use EMT conduit and run an EGC within it (and they are tied together at each end of the conduit), you meet the intent of this code?

I guess I am trying to figure out how this is any different than what we normally do. We (firm I work in) typically only allows EMT in the interior with flex for special uses only... very rare for nonmetallics except for service entrance type conduits. We also always call out for a separate green grounding conductor.

 

[raider1]

So as long as you use EMT conduit and run an EGC within it (and they are tied together at each end of the conduit), you meet the intent of this code?

Correct.

I guess I am trying to figure out how this is any different than what we normally do. We (firm I work in) typically only allows EMT in the interior with flex for special uses only... very rare for nonmetallics except for service entrance type conduits. We also always call out for a separate green grounding conductor.

The use of standard MC cable is prohibited in patient care areas and a lot of companys use cable wiring methods instead of EMT for buildings due to the lesser labor costs of cable.

Chris

 

[hurk27]

So as long as you use EMT conduit and run an EGC within it (and they are tied together at each end of the conduit), you meet the intent of this code?

I guess I am trying to figure out how this is any different than what we normally do. We (firm I work in) typically only allows EMT in the interior with flex for special uses only... very rare for nonmetallics except for service entrance type conduits. We also always call out for a separate green grounding conductor.

What you may normally do and what the code minimum is are two different things, code allows EMT to be run outside, code allows AC/MC cable to be used in many places, but in patient care areas where there is a increased danger to a patient, these allowances are much more strict, and 517.13 has requirements that don't allow other wiring methods that are allowed in other areas of the code.