How dangerous DC & AC

How dangerous electricity is?

Output process of solar photovoltaic power plant deals with both AC (Alternating Current) and DC (Direct Current) power of the rage 390-430VAC and 360-1500VDC respectively in various types and capacity.  It is assumed that less than 50V is a touch potential that is considered to be “safe” from the standpoint of an electric shock hazard[1]. If any energized electrical conductors or circuit parts operating at 50V or more are not placed in an electrically safe work condition, then other safety-related work practices must be used to protect the operator/ employee from the electrical hazards involved. For wet locations even 30V can cause hazard, the impedance of the body is less when wet, and thus voltage across a wet body can produce more current through the body than would be the case if the body was dry [2].

In case of photovoltaic power plants the situation is more serious; chances of electrocution are more compared to any other power generating methods. Factors which making the PV power plant more accident-prone region are as follows:

•    Deals with AC and DC

•    DC can be above 900 - 1500 VDC in case of power plant of capacity ≥5kW

•    Output AC - 230V (1ɸ) or 415V (3ɸ)

•    100% isolation of generating part is not possible : a single PV module can also produce 30 to 45VDC, and ~7-9A with daylight

•    Wide spread area and direct contact with generating part

•    Exposed to atmosphere makes high penetration of wet condition

•    Electrocution on rooftop may lead to other hazards like fall from height

Hot-work flow chart [3]

Which is more dangerous AC or DC ?

This is a common doubts to all, Which is more dangerous,  AC or DC?. The answer is both are dangerous; but technically speaking in case of same amount of power AC can cause more impact on human body. This is because of following reasons

       Magnitude

The effect of the currents on the body is a direct result of the excitatory actions of its magnitude  specifically, the actual making and breaking of the current itself. Such excitatory actions include nerve / muscle stimulation, induction of cardiac atrial or ventricular fibrillation, and more.  For a measured voltage for V in case of DC and AC, peak value of DC is same as measured value. But AC measured value is RMS value and it is 1.414 times measured RMS value. Following figure shows the magnitude difference between AC and DC.

Body resistance and capacitance Human body has both a resistance and a capacitance in parallel configuration.

Thus when electricity goes through both of these paths a significant increase occurs of the overall current entering the body.

When DC current passes through the body, only R_s path will get enabled this is because of the blocking effect of capacitor on DC. Capacitive impedance (X_c) is inversely proportional to frequency, since the frequency of DC is zero X_c become infinity; that is why capacitor blocking DC. At AC frequency is greater than zero. It’s usually above 50Hz thus the X_c is significantly lower than with DC Thus AC is more likely to pass through the Capacitive property of the skin.

References

[1]   Standard for Electrical Safety in the Workplace NFPA 70E, 2000 edition

[2]  Robert Backstrom, David A. Dini, P.E Firefighter Safety and Photovoltaic Installations Research Project, Underwriters Laboratories Inc., Issue Date: November 29, 2011

[3]  John Cadick, Mary Capelli-Schellpfeffer, Dennis Neitzel, Electrical Safety Handbook, McGraw-Hill, 3^rd edition, 2005

[4] Kyle Hill, General Science, How much electricity can you take on before you die? [Online Available]: https://sciencebasedlife.wordpress.com/2012/02/02/how-much-voltage-can-you-take-on-before-you-die/ Accessed on 31/05/2018