# The earth rod next to the generator: what is it for? > Neutral reference in TN-S, fault current path in TT: the role of a generator's earth rod, the French NF C 15-100 rule and how to measure it. Source: https://www.darkside-energy.com/en/did-you-know/generator-earth-rod/ · Dark Side Energy · Updated: 2026-11-09 9 November 2026 · 3 min read - The rod connects the installation supplied by the generator to the ground: its role depends on the earthing system chosen. - In TN-S, it is a reference and the fault current returns through the protective conductor. In TT, it is on the fault current path: the French NF C 15-100 standard requires RA × IΔn ≤ UL so that the RCD trips. - An earth rod is measured, with an earth resistance tester. A [generator](https://www.darkside-energy.com/en/glossary/#groupe-electrogene) is a stand-alone source: nothing connects its neutral to the ground unless this is planned. Around it, at an event, metal structures, barriers or the installation of a nearby building are in contact with the ground. The [earth electrode](https://www.darkside-energy.com/en/glossary/#prise-de-terre), made of one or more rods driven into the ground, gives the installation the same reference: the potential of the ground where the public and the crews stand. ## Two roles, depending on the system The earthing system is chosen at the design stage, and the rod does not play the same role in each. - **In TN-S**, the generator neutral is connected to its frame and to the protective conductor (PE), and the rod connects this point to the ground. The installation remains TN-S: its exposed metal parts are connected to the generator neutral through the protective conductor, which is what defines a TN system. In the event of a fault, the current returns to the generator through the protective conductor, not through the ground. - **In TT**, the exposed metal parts of the equipment are connected to their own earth electrode. The fault current flows through the ground back to the source: too weak to trip a circuit breaker, it is detected by the [RCD](https://www.darkside-energy.com/en/glossary/#differentiel-30-ma). The earth electrode resistance must then stay low enough for the RCD to trip before a dangerous voltage appears on the metal parts. ## What rule in France? In TT, the French NF C 15-100 standard sets the rule RA × IΔn ≤ UL: the earth electrode resistance (RA) multiplied by the RCD rated residual current (IΔn) must not exceed the conventional touch voltage limit (UL). The more sensitive the RCD, the higher the permitted resistance: the earth electrode and the RCD are chosen together. The voltage limit to use depends on the conditions of the installation: the study sets it, and the maximum earth electrode value follows from it. In TN-S, the rod is not what makes the protective device trip: the fault current returns through the protective conductor. ## An earth rod is measured The resistance of an earth electrode depends mainly on the ground: its nature, moisture and temperature. It is measured on site, once the rod is driven in, with an earth resistance tester. And when equipment supplied from different generators is used side by side, the NF C 15-100 standard sets the principle: exposed metal parts that can be touched at the same time must be connected to the same earth electrode. The generators' earths are therefore interconnected. > An earth rod is not driven in for show: its role depends on the earthing system, and its value is measured. The earthing system of an installation supplied by generators is decided upfront: this is the work of our [engineering office](https://www.darkside-energy.com/en/engineering/). A project running on generators? The power assessment works out the power to plan for from the list of your equipment. [Do a power assessment](https://www.darkside-energy.com/en/power-assessment/) [Cable on a drum: why unwind it fully?](https://www.darkside-energy.com/en/did-you-know/cable-on-a-drum/) ## A project? Let's talk power. Venue, dates, known requirements: tell us about your event and we will get back to you quickly. [Request a quote](https://www.darkside-energy.com/en/contact/) [contact@darkside-energy.com](mailto:contact@darkside-energy.com)