The Relationship Between Residual Current Devices (RCD) and Grounding

The formation of electric current depends on the existence of a potential difference between two points. In electrical accidents, the contact of the phase conductor with another conductive surface causes the formation of a fault current. This fault current can lead to death, injuries, and accompanying major financial damages depending on the short-circuit power. Grounding measurement will limit the fault current that will occur by regulating the contact voltage exposed in such a case.

Having a good grounding installation does not mean there is no vital risk. To explain with an example;

Let’s assume we have a machine with 100 Amperes nominal current. There is no need for magnetic switches, residual current relays, and fuses for this machine to work. It works easily when we connect it to the LV side of the transformer via a conductor. It can continue to work by drawing a current equal to the nominal current value.

mechanical technician operative of cnc milling cutting machine center at tool workshop

Thermal magnetic switches, fuses, or residual current relays we use are protection elements.

[Image of electrical grounding protection diagram]

Let’s assume we installed a grounding line with a value of 1 ohm to the body of this machine with 100 Amperes nominal current. The average resistance value of a human is between 2000 – 3000 ohms. Let’s take the human resistance as 3000 ohms and assume there is contact with the machine body.

As a result, a high short-circuit current will occur with the contact of any of the phase conductors to the machine body. For example, let’s assume this short-circuit current is 3001 Amperes. At points where the main distribution and conductor cross-section are very large, the short-circuit current is at much higher values. This high short-circuit current will be shared inversely with their resistances between the human in contact with the machine body and the grounding line. That is, a large current will pass through the one with small resistance, and a small current will pass through the one with large resistance.

Human resistance : 3000 Ohm

Grounding resistance : 1 ohm

Maximum short-circuit current : 3001 Amperes

Current to pass through human : 1 ampere

Current to pass through grounding line : 3000 amperes

As a result, the 1 ampere current that will pass through the human poses a hundred percent vital risk.

If the grounding line is strengthened further, the resistance of the line will decrease, so the short-circuit current will rise to a higher value than it is. Therefore, the current share falling to the human will increase even more.

Ultimately, a good grounding alone does not protect the human, and no grounding saves lives in a place where there is no protection element.

If the magnetic switches, fuses, or other equipment we buy by paying high costs do not perform the protection function, they mean nothing. It even means that the Machines are directly connected to the transformer.

To explain with another example;

Let’s have a 10 Ampere machine. If we want to add a protection element to the line this machine is fed from, we must not forget two important points.

1. The nominal current value of the protection element must be equal to or greater than the load current.
2. It must trip in phase-phase, phase-neutral, or phase-ground short circuits that may occur on this line. It must protect the line, the machine, and the human.

Let’s assume we are using a 16 Ampere C series fuse. Our mains voltage is 230 volts. The instantaneous tripping current of a C series fuse in case of a short circuit is 10 times its nominal current. That is, it will trip at 160 amperes short-circuit current and above.

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Let the grounding resistance of the machine be measured as 1.5 ohms. The protection element protecting itself determines the limit value of a machine’s grounding resistance. That is 230 volts / 160 amperes : Nominal voltage / protection element tripping current : 1.43 ohms.

As a result, when the calculated value 1.43 is smaller than the measured value 1.5, the C 16 Ampere fuse does not trip for a possible short circuit to this machine body, meaning it does not protect.

Therefore, the operator working on this machine is under great risk.

As Yılkomer, we provide professional services for this field in all enterprises.

We inspect not only the grounding resistances of enterprises but also their protection equipment one by one and detect their non-conformities.

We design an effective life protection system by intervening in protection equipment.

If the existing protection equipment consists of new generation (residual current controlled) protection elements, we inject the life protection system by directly intervening in them. If the protection elements are old, we design the system by replacing them with new generation (residual current controlled) protection elements.

While doing this process, we make the tripping currents of thermal magnetic switches, which have very high tripping currents, sensitive from 30 milliamperes to 30 amperes by using the toroid + relay + shunt trip coil combination. We calculate the tripping current value of the thermal magnetic switch by measuring the existing residual current amount.

At the end of these operations, we make a magnetic switch with a 10 kiloampere tripping current as sensitive as a residual current relay .

The importance of protection elements is stated in the regulation on grounding in electrical installations and the regulation on electrical indoor installations, and no report without protection elements is accepted by the Ministry of Labor and Social Security.

Regardless of the type of system (TT, TN-S, TN-C, TN-C-S, IT), protection elements must be active. Grounding does not save lives. It is the protection element that saves lives.

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