DC microgrids in buildings — earthing choice and the polarity trade-off on a DC busbar

The guide on the DC side of a BESS covers overcurrent protection and cable sizing between battery modules and a single inverter. Increasingly, however, DC voltage is used not only internally within one device, but as an actual DC microgrid busbar within a building itself — for example a 350-380V DC busbar that directly serves a PV installation, a battery storage system, EV charge points and (in data centres) server power supplies, without having to convert back and forth to AC each time. This article covers an aspect that this architecture requires a fundamentally different approach to than an ordinary AC installation: how such a DC busbar is earthed, and why that choice is not simply a DC translation of the familiar TN/TT/IT classification.

Why TN/TT/IT does not translate one-to-one to DC

The TN, TT and IT earthing systems (see the guide on earthing systems) are defined for an AC network with a neutral conductor. A DC busbar has no neutral, but does have two poles (+ and −, possibly with a mid-point tap on a symmetric ± busbar). The underlying principles — how the system reacts to a first fault, what determines the touch voltage — carry over, but the concrete implementation differs:

Solidly earthed pole (comparable to TN/TT): one of the two poles is permanently connected to earth. A fault on the non-earthed pole then produces a large fault current that overcurrent protection can clear — comparable to a TN/TT fault. A fault on the already-earthed pole itself, however, has no consequence for the fault current (that pole is already at earth potential), which complicates detecting that specific fault.
Ungrounded busbar with insulation monitoring (comparable to IT): neither pole is permanently earthed; an insulation monitoring device (IMD) continuously monitors the insulation resistance of both poles to earth, just as with the IT system. A first fault then produces no large fault current and the installation can (as with an AC IT system) remain in operation until the fault is repaired — relevant for critical DC applications where operational continuity outweighs immediate disconnection on a first fault.

Note: an IMD designed and calibrated for an AC network is not automatically suitable for a DC busbar — most IMDs' measurement principles rely on AC coupling capacitors and can give incorrect or slow fault detection on a DC busbar. A DC application requires an IMD explicitly qualified for direct current.
Mid-point earthed ± busbar: with a symmetric busbar (for example ±175V around a virtual zero for a 350V system), the mid-point is earthed. This halves the maximum touch voltage between each pole and earth compared with a single-pole-earthed busbar of the same total voltage.

The polarity trade-off: which pole to earth on a solidly earthed busbar

On a solidly earthed two-pole DC busbar, a choice must be made: which of the two poles is earthed? This turns out not to be a trivial choice, and different considerations even point in opposite directions:

From a safety perspective (IEC 60479-1): direct current flowing upward through the body (conventional current direction from feet to head) is assessed by the heart as more dangerous for ventricular fibrillation than the same current in the opposite, downward direction — see also the guide on the IEC 60479-1 current-time curve, which covers the same standard for the usual AC thresholds. For this reason, IEC 60479-1 considers earthing the negative pole preferable on a two-pole DC busbar: on a fault on the (then non-earthed) positive pole, the fault current through a body touching that pole predominantly flows in the less unfavourable direction.
From a corrosion perspective: electrochemical (galvanic) corrosion under direct current predominantly occurs at the pole that drives current into the earth. If the negative pole is earthed, any corrosion occurs at the earth electrode itself; if instead the positive pole is earthed, that corrosion risk shifts away from critical wiring toward the (easier to maintain or replace) earth electrode — the underlying reason why traditional telecom DC systems (the well-known "-48V" system) earth the positive pole, despite the safety consideration described above.

Note: these two considerations point in opposite directions, and which one is decisive depends on the application (the likelihood of people being exposed to a fault on the non-earthed pole versus the installation's sensitivity to corrosion damage over its lifetime). This is a design decision to be made per project, not a universally prescribed rule.

Touch voltage limit: higher for DC, but not unlimited

As covered in the guide on the conventional touch voltage limit UL, the conventional touch voltage limit for direct current under dry, normal conditions is 120V (versus 50V for alternating current). That does not mean a 350-380V DC busbar is acceptable without further measures: that voltage lies well above even the elevated DC limit, so automatic disconnection on a fault (or a comparable protective measure) remains just as mandatory as for an AC installation of comparable voltage.

Practical relevance

When designing a DC microgrid busbar in a building, the earthing choice — solidly earthed (which pole), mid-point earthed, or ungrounded with a DC-capable IMD — must be a deliberate, documented design decision, just as the choice between TN, TT and IT is for an AC installation. For the associated overcurrent protection and cable sizing on either side of that busbar, the [guide on the DC side of a BESS](/guides/nen-1010/bess-gelijkstroom-dc-beveiliging-kabeldimensionering) remains the starting point.

Common mistakes

Applying an AC IMD to a DC busbar without modification — the measurement principles are often not automatically suitable for direct current and can leave a fault missed or reported too late.
Using the elevated DC touch voltage limit (120V) as a reason to protect less strictly on a busbar that itself lies well above that limit (such as a 350-380V busbar) — the elevated limit changes nothing about the need for disconnection on a fault at that voltage level.
Copying the polarity choice (which pole to earth) from another application (for example blindly following the telecom convention of an earthed positive pole) without assessing the project's own safety and corrosion considerations.
Not accounting for the specific impedance requirements of DC switchgear on a solidly earthed busbar — see the guide on the DC side of a BESS for the background on why DC switchgear is not simply AC switchgear.