Micro Shock

Status
Not open for further replies.
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Is code mandated grounding and bonding enough to stop micro shocks? Can standing neutral to ground faults cause them?

1579506856368.png


ELECTRICAL DISTRIBUTION SYSTEMS IN HOSPITAL - ppt download

This Week Hospital Electrical System: Introduction General power and lighting system Distribution of electrical power system Electrical Hazards in Hospital Macrocshock Microshock Leakage current Electrical Safety Cord and Standards Protection Against Shock Isolated Power distribution Ground...
slideplayer.com
 
Yes, and no.
1. Phase to ground shorts can elevate the neutral and ultimately through the resistance of the POCO service neutral and the GES if a local earth ground is accessible (such as water pipes, structural steel, etc., even when NEC mandated grounding and bonding rules are followed.
2. Phase to neutral shorts can cause microshock potentials IF NEC grounding and bonding standards are not followed, AND can cause microshock potentials even when NEC mandates are followed if the entire service neutral voltage is raised, elevating the ECG potential to distant earth at the same time. The microshock risk occurs when a different source of local ground is accessible, such as via water pipes, structural steel, concrete floors, etc.
3. The specific bonding standards for pools are designed to eliminate (or greatly reduce) the chance of microshock potential differences from any offset of bond system potential from distant earth/EGC potential, regardless of the source. Other areas of buildings and yards do not have that extra equipotential bonding of all surfaces which can be touched, including local dirt.
4. All of the above risks can be mitigated by proper overcurrent and ground fault protection.
 
Regarding #2, doesn't the NEC as written eliminate all remote earth points in an OR?

In general- my understanding is that during a fault the EGC will become a resistive divider- making for voltage differences within the bonding system itself? Over current and GF would shorten that duration.
 
One other issue is that the health effects of 'microshocks' are unclear.

The image in the OP states a maximum safe threshold of 10uA, well below the threshold of sensation. GFCIs trip at 600x this level.

There has been research into the health effects of 'contact current' as a possible mechanism for the (possible but not proven) association of 'EMF' exposure and leukemia. If you want to explore this you will need to wade through a bunch of EMF woo. (I believe that there is a real possibility of non-thermal effects from EMF exposure, but there is a huge amount of bs and fear mongering in any discussion of such, so I don't spen much time on it.)

-Jon
 
winnie said:
One other issue is that the health effects of 'microshocks' are unclear.

The image in the OP states a maximum safe threshold of 10uA, well below the threshold of sensation. GFCIs trip at 600x this level.

There has been research into the health effects of 'contact current' as a possible mechanism for the (possible but not proven) association of 'EMF' exposure and leukemia. If you want to explore this you will need to wade through a bunch of EMF woo. (I believe that there is a real possibility of non-thermal effects from EMF exposure, but there is a huge amount of bs and fear mongering in any discussion of such, so I don't spen much time on it.)

-Jon
Click to expand...


Rather worse than that- it takes micro amps to send the heart into fibrillation. At least what Mike Holt taught me.
 
The article does not mention the requirement of the equipotential in patient care rooms to be less than 0.1 ohm which solves the problem of multiple earth points. This equipotential must be tested with apparatus as shown in annex A of NFPA 99 prior to commissioning the room(s) and periodically there after.

Roger
 
mbrooke said:
Rather worse than that- it takes micro amps to send the heart into fibrillation. At least what Mike Holt taught me.
Click to expand...

If you are talking whole body shock, eg from an arm to a leg via the torso, then the current expected to cause fibrillation is in the 10s of milliamps.

I bet you wouldn't even feel a 10 microamp current from hand to foot.

I can believe the number is much lower if you are talking of a patient care scenario with direct electrical connection to the heart.

-Jon
 
roger said:
The article does not mention the requirement of the equipotential in patient care rooms to be less than 0.1 ohm which solves the problem of multiple earth points. This equipotential must be tested with apparatus as shown in annex A of NFPA 99 prior to commissioning the room(s) and periodically there after.

Roger
Click to expand...

This
 
roger said:
The article does not mention the requirement of the equipotential in patient care rooms to be less than 0.1 ohm which solves the problem of multiple earth points. This equipotential must be tested with apparatus as shown in annex A of NFPA 99 prior to commissioning the room(s) and periodically there after.

Roger
Click to expand...


Thanks :)
 
winnie said:
If you are talking whole body shock, eg from an arm to a leg via the torso, then the current expected to cause fibrillation is in the 10s of milliamps.

I bet you wouldn't even feel a 10 microamp current from hand to foot.

I can believe the number is much lower if you are talking of a patient care scenario with direct electrical connection to the heart.

-Jon
Click to expand...


Agreed. Remember though that the body is a near infinite number of resistors in series parellel (mesh). 50ma hand to foot would be much less across the heart.
 
Status
Not open for further replies.
Top