Digital Shore Hardness Tester Manual

Electric shock is caused by flash strick as flashing lighting touch the household electrical wiring, or accidently broken wire touch electrified body , easily lead to electrical equipment damage and cause personal safety accident.

Protection against electric shock is a important method to prevent electric shock. There are several ways:Automatic power off, II equipment, Insulation equipment, electrical isolation; equipotential connection without GND.Automatic power off become the nomalest way to prevent electric shock because of it’s simpleness and operability.As a result, residual current devices with automatic power off technology are more and more widely used.

Residual current devices works by receiving signal,so it works also under other electromagnetic interference signal which called mal operation. When the power swith turn on, it comes out the impact signal cause to the mal operation.The sum of multi-branch electric leakage may lead to override maloperation.When neutral line iterative earthing may cause residual current devices comes to shunt miss operation. When the power leakage happens to the one used by residual current device, it comes miss operation.

As can be seen from the above analysis , the residual current devices have mal operation and miss operation situation in actual use. This article describes the basic structure and principles of residual current devices , focused on effects of the Earth Current Frequency and Distortion on Residual Current Devices in the protection against electric shock.

 

1.Electric Shock

Most electrocution death and equipment damage accidents are caused by electric shock . Generally speaking, the electric shock accident basically means in terms of electric shock . In accordance with the state of the electrical equipment , electrical shock can be divided into direct contact and indirect contact type.The former is a normal state of electric shock when human touch the charged body,The latter is a shock occurs when people touch the accidently charged body under broken situation but no uncharged normally. Therefore , the former is known as electric shock of normal state , the latter is known as electric shock under breakdown conditions.

According to the way of how people body touch the charged object and the way of how the electric current go through people body, shock can be divided into single-phase electric shock, two-phase electric shock and step voltage electric shock.

single-phase electric shock is mean human body touch the electric conductor in earth or landing, so the partial body of human body touch the charged object lead to electric shock accident. And most shock accident belongs to such kind. The dangerous degree of single-phase shock is related to the operation mode of the electric network. Normally, the single-phase of landing electric net work is more dangerous than UNGND electric network. Two-phase electric shock means the accident that two places in human touch two-phase charged dody , it’s very dangerous. When charged body landing , it’s current go to earth lead to the voltage of GND earth decline. When people closed to earth electrode, the voltage appeared between feets is called step voltage which lead to step voltage electric shock accident. It may appears high step voltage in high voltage broken GND point or beside earth device when large current go through.

IEC 60479-1 defines the four region of A / t relationship , described the morbid physiological effects within each region of the body. As shown in Picture 1 , the horizontal axis represents the current figure flowing through the body , the vertical axis represents current sustained time. Every area shows:AC- 1 zone indicates no perception ; AC-2 area expressed perception ; AC-3 region indicates the presence of reversible effect of muscle contraction ; AC-4 zone may indicate irreversible effects ; AC-4-1 zone may represent cardiac fibrillation up to 5%; AC-4-2 zone represents cardiac fibrillation up to 50%; AC-4-3 zone represents cardiac fibrillation can be more than 50% ;

Each curve in following picture shows : Curve A represents the current perception threshold ; Curve B represents the muscle response threshold ; Curve C1 represents the 0% probability of ventricular fibrillation threshold ; Curve C2 represents a 5 % chance of ventricular fibrillation threshold ; C3 curve represents 50% probability of ventricular fibrillation threshold. Take Curve C1 as an example , when the current go through human body , if the current exceeds 30 mA, the human body is likely to be fatal electric shock unless the current can be cut off in a relatively short period of time.