ᐅ Should the underground cable to the garden be additionally protected?

Hello Forum,

We have a 5x2.5mm (approx. 5x0.01 inch²) underground cable running from the house wall to the garden. Before I consider how and where to lay or distribute this cable, I’m wondering if a separate circuit breaker and residual current device (RCD) are useful or even necessary. Actually, “separate” isn’t quite correct since the cable branches off from the living room circuit. It’s not ideal, but that’s how it is now – I didn’t account for this during the house construction. The living room circuit is protected by its own 16A circuit breaker as well as an RCD with a 30mA trip threshold. So if there were an uncontrolled current flow in the garden, only the living room (and a few other areas connected to the same RCD) would lose power. The heating and other rooms are separately protected by different RCDs.

Since the RCD in the living room should quickly detect fault currents in any garden electrical installation, my question is: would a portable RCD device installed outdoors between the supply cable and the garden distribution actually provide any benefit, apart from possibly keeping the living room powered in case of a fault?

From my understanding, a circuit breaker at this point would only be of limited use. With the same trip threshold as the main panel breaker – 16A – there is no guarantee the garden breaker would trip without simultaneously tripping the upstream breaker in the main panel. Following this logic, only a circuit breaker with a lower rating, for example 10A, would make sense here.

In short: does having a downstream RCD and/or circuit breaker in the garden provide any tangible increase in safety?

A related question is where to install these protective devices. Practically, only a weatherproof box suitable for damp locations, mounted at ground level, seems feasible. I’m not sure if such a box exists or is even permitted.

To ensure selectivity, the upstream RCD should have a trip current three times that of the downstream device, so 10mA.

However, whether this is still suitable for a garden installation, I’m not sure. It depends on what equipment you plan to operate and the leakage currents of the devices.

In some cases, it might make sense to build a control panel and protect each device individually with 10mA RCDs.

But of course, only if you are a qualified electrician; otherwise, please do not attempt this yourself.

Who or what do you want to protect? Users, cables, devices, or to prevent a ground fault from turning off the living room lights?

As a general rule, use a rating two steps lower than the upstream circuit breaker. So rather 6A than 10A. Specifically, you would need to look at the trip characteristics, meaning the minimum trip current of the larger breaker and the maximum trip current of the smaller one.

xMisterDx schrieb:

As a general rule, use a breaker two steps smaller than the upstream breaker. So rather 6A (6.6A) than 10A (10.6A). Specifically, you would need to look at the trip characteristics—meaning the minimum trip point of the larger breaker and the maximum trip point of the smaller one.

And what exactly is the benefit of that?
What does it protect against?

Installing a 6A (6.6A) breaker just means you can no longer operate larger appliances.

First of all, it is important to ensure personal safety for the garden installation through circuit breakers and RCDs (residual current devices) in the distribution board. It would be a nice-to-have if the living room does not lose power in case of a fault. Having a circuit breaker in the garden would at least allow disconnecting the garden power for troubleshooting without cutting off electricity to the living room at the same time.