RCD instead of an introductory machine: why is it dangerous and what does the PUE say?

Introduction: why the question is relevant for car owners

If you do electrical work in a garage, workshop or car service center, sooner or later you will face a dilemma: is it possible to save space in the panel by replacing input circuit breaker on RCD (residual current device)? At first glance, both devices are mounted on a DIN rail, have similar dimensions and even look similar. But in practice, such a replacement is fraught short circuits, fires and failure of expensive equipment - from welding machines to charging stations for electric vehicles.

In this article we will analyze three key reasons why an RCD cannot fully replace an input machine, even if its rated current coincides with the calculated network load. Based on PUE 7th edition (Electrical Installation Rules) and the experience of auto electricians, we will show you how to correctly combine these devices to protect both wiring and life. And at the end - answers to frequently asked questions with examples from real cases.

How is an RCD fundamentally different from an automatic machine?

The main difference lies in operating principle:

• 🔌 Circuit breaker (AB) reacts to overcurrent - for example, in case of network overload or short circuit. It breaks the circuit, preventing wire melting and fire.
• ⚡ RCD (residual current device) only trips when current leakage to ground (differential current). It does not protect against overloads or short circuits!

Simply put, if a short circuit occurs in the welding machine in the garage Lincoln Electric or compressor overload, The RCD will not turn off the power - it's just not designed for that. But the machine (for example, ABB S200 or IEK BA47-29) will work in a split second.

In addition, the RCD does not have time-current characteristics (like B, C, D in automatic machines), which determines how quickly it will respond to a current surge. This is critical for networks with high inrush currents - for example, when starting asynchronous motors in car repair bays.

What the PUE says: regulatory framework

B PUE 7.1.71 clearly stated: “Control and protection equipment must be installed at the entrance to the building”. By “protection” we mean precisely overcurrent protection (i.e. circuit breaker) and not from leaks. The RCD is not even mentioned as an alternative in this paragraph.

Moreover, clause 7.1.73 requires that “a control device was installed at the entrance to an apartment or individual house” (switch or automatic) with the possibility complete blackout. The RCD cannot perform this function because:

• 🔧 Does not have a mechanism for forced opening of the circuit (unlike switches).
• 📉 Does not protect against overloads - only against leaks.
• ⚡ Not designed for breaking capacity (switching resistance) during short circuit.

The exception is difavtomats (RCBOs), which combine the functions of an automatic machine and an RCD. But they must also have corresponding denomination and response characteristic (usually C or D for garages).

Real risks: what will happen during replacement

Imagine the situation: in the garage it is connected inverter welding machine at 200 A, and at the input instead of a machine there is a 40 A RCD. In case of a short circuit in the cable:

1. Short circuit current can reach thousand amperes (depends on the circuit resistance).
2. RCD won't turn off, since it does not respond to overcurrents.
3. The wiring heats up to copper melting point (1083°C), a fire occurs.
4. If the RCD still burns out due to thermal effects, the circuit may stuck in a closed state (especially for cheap models like IEK or TKD).

According to the Ministry of Emergency Situations, 23% of fires in garages in 2023 occurred precisely because of improper protection of input circuits. Moreover, in 80% of cases the fault was homemade circuits with RCD instead of automatic machines.

What to do if the RCD is already at the input?

If you discover that an RCD is installed at the entrance to a garage or workshop, immediately:

1. Turn off the power at the input (if there is a switch in front of the RCD).

2. Check the cross-section of the input cable - if it is less than 6 mm² for copper or 10 mm² for aluminum, the risk of overheating is extremely high.

3. Install a circuit breaker in front of the RCD with a rating corresponding to the cable cross-section (see table below).

4. If there is no space in the panel, replace the RCD with a difavtomat with a suitable time-current characteristic (C or D).

When can an RCD be installed for input? Exceptions to the rules

Yes two cases, when an RCD is allowed to be installed at the input, but with strict reservations:

1. After the machine: RCD is installed after input machine and plays the role fire protection (with leakage current 100–300 mA). This scheme is common in Europe and is recommended for garages with a damp environment.
2. Paired with a switch: If an RCD is installed in front of switch-load (for example, ABB OT40F3), which can break the chain manually. But this does not protect against short circuit!

An example of a correct layout for a garage:

Input cable → Switch → Automatic 40A (C) → RCD 40A/100mA → Group automatic circuit breakers

Please note: The rated current of the RCD should be one step higherthan that of a machine (for example, machine 25 A + RCD 40 A). This is due to thermal characteristics devices.

Input cable cross-section, mm²	Max. circuit breaker current, A	Minimum RCD current, A	Recommended characteristic
4 (copper)	25	40	C
6 (copper)	32	40	C or D
10 (aluminum)	40	63	D
16 (copper)	63	80	D

Checklist: how to properly protect your garage entrance

If you decide to update the dashboard in a garage or workshop, follow this algorithm:

Calculate the total load (welding machine, compressor, lighting, sockets)

Select an input circuit breaker with a rating 20–30% higher than the rated current (for example, for a load of 30 A, take a 40 A circuit breaker)

Install a machine with characteristics C (for mixed loads) or D (if there are engines)

Add RCD after machine with a leakage current of 100–300 mA (fire protection)

For wet rooms, use an RCD with a leakage current of 10–30 mA per socket groups

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Calculation example for a typical garage:

• 🔥 Welding machine Resanta SAI-220: 200 A (peak current), but average consumption is 25 A.
• 💡 Lighting (10 lamps of 50 W): 0.5 A.
• 🔌 Tool sockets: 10 A (stock).
• ⚙️ Compressor Fubag: 15 A.

Total: 25 + 0.5 + 10 + 15 = 50.5 A. The optimal input machine is 63 A (characteristic D), RCD - 80 A/100 mA.

Myths and misconceptions about RCDs at the input

On the Internet you can find advice like: “Install a 63 A RCD instead of a machine - and save space in the panel!”. Let's look at the most common myths:

⚠️ Attention: RCD doesn't have arc extinguishing chamber, which is found in machines. When the chain breaks under load, it occurs electric arc, which can melt the contacts and cause a fire. The machines are specially structurally protected from this.

• 🤖 Myth 1: “The RCD trips during overload if the current exceeds the nominal value.”

  Reality: The RCD may overheat and fail, but won't turn off circuit when overloaded. Its task is only to leak.

• 💰 Myth 2: “Difavtomatic is expensive, but RCD + automatic is cheaper.”

  Reality: Difavtomat ABB DS201 for 40 A it costs ~3,500 ₽, and a pair of automatic + RCD (for example, IEK) — ~2,800 ₽. But saving 700 ₽ is not worth the risk of fire.

• ⚡ Myth 3: “I have a 100 A RCD, and nothing lights up.”

  Reality: Until a short circuit occurs, you will not know whether the protection will work. And when it happens, it may be too late.

Another dangerous misconception: “The RCD protects against electric shock”. In fact, it only protects against leaks, but not from direct contact with the phase. For complete safety you need TN-S system with proper grounding.

FAQ: answers to frequently asked questions

Is it possible to install an RCD in front of the machine?

Technically possible, but pointless. The RCD will not protect the machine and wiring from overloads. In addition, during a short circuit the machine may not work due to the fact that the RCD will stick in a closed state. Correct order: input → automatic machine → RCD → group automatic machines.

What happens if you install an RCD with a rating lower than that of the machine?

For example, a 40 A automatic machine + a 25 A RCD. In this case, the RCD will burn at a load above 25 A, since it is not designed for prolonged current loads. Always take an RCD with a rating one step higherthan that of an automatic machine.

Is it possible to use an RCD instead of a machine in the charging circuit of an electric vehicle?

Absolutely not! Charging stations (eg Tesla Wall Connector or ABB Terra AC) require mandatory overcurrent protection. B PUE 7.1.82 it is stated that the power circuits of electric vehicles must be protected circuit breakers with characteristic C or D + RCD with leakage current no more than 30 mA.

How to check that the input is a machine and not an RCD?

Inspect the device:

• The machine will indicate time-current characteristic (B, C, D) and breaking capacity (for example 4500 A).
• On RCD - only rated current and leakage current (30 mA, 100 mA, etc.), as well as a button TEST.

In addition, the machine is triggered when a phase and zero are short-circuited, and the RCD only triggers when there is a leak to ground.

Which RCD should I choose for a garage with a damp environment?

For wet rooms (for example, if a car is often washed in the garage), it is recommended:

• RCD with leakage current 10 mA for socket groups.
• RCD with leakage current 100 mA at the input (fire protection).
• Degree of protection IP44 or higher (for example, Legrand DX³ IP65).

Be sure to combine with a machine and check the grounding!