Isolated Ground DC

Most of our DC systems have the negative side of the system, what we have called circuit ground, connected to earth ground – i.e., the water surrounding the boat. The ABYC also calls earth ground source ground; we will use this term. 

We may have various other circuits also connected to source ground. These will include the AC grounding circuit if we have ABYC-compliant AC circuits, any bonding or lightning protection circuits, and perhaps electronic grounding circuits. The connection to source ground holds these circuits at a common potential. This is done to prevent potentially dangerous voltage differences arising between different metal objects on the boat (e.g., given a fault in an AC circuit or a lightning strike), or to connect submerged metal objects to a sacrificial anode, or to mitigate against stray current corrosion, or to minimize radio frequency interference, or for all these reasons. 

We cover the various grounding circuits in this module of our Advanced Marine Electrics program and provide a more detailed explanation of the reasons for these circuits in our two AC modules, and the corrosion and lightning modules.

In some circumstances, primarily for corrosion control, it is desirable to isolate the negative side of a DC system from source ground. In this case we have what the ABYC sub-committee calls an isolated DC system and what the ISO calls a fully insulated two-wire DC system. The full definitions are:

• ABYC sub-committee: ‘Isolated DC system (i.e., fully isolated two-wire DC system, ungrounded, insulated, or floating ground DC system) – a system in which both positive and negative sides are intentionally isolated from source ground (earth).’
• ISO: ‘Fully insulated two-wire DC system. System in which both positive and negative poles remain isolated from the ground (earth), e.g., not connected to the water through a metallic hull, the propulsion system or earthed through the AC protective conductor.’

Note that it is possible to have both grounded and ungrounded DC systems on the same boat. For example, some high-pressure common rail diesel engines (see our forthcoming diesel lessons) require isolated cranking and battery charging circuits while the house batteries and circuits may be grounded. Conversely, an engine cranking circuit may be installed as a grounded DC system while the house batteries on the boat are installed as an isolated (ungrounded) DC system. Some bow thrusters, with their own battery that is not connected to other batteries on the boat are installed as an isolated ground system while the house batteries are grounded. And almost all electric propulsion systems above 50 volts are installed as isolated ground systems while the rest of the boat’s DC systems are typically grounded. 

In all such installations it is important to maintain the separation of grounded and ungrounded systems, which is sometimes not easy to do. In this lesson we will explore the special installation requirements of isolated DC systems and look at mechanisms to check that they are isolated.

Defining the core components

Let’s first recap the essential differences between a grounded DC system and an isolated ground DC system.

The negative side of a grounded DC system is wired back to some common ground point which is then connected to battery negative. On boats with very limited DC systems, this common ground point may be the engine negative terminal but with more substantial DC systems it is typically a busbar which the ABYC calls the DC main negative bus. The negative side of an isolated ground DC system is not wired to the engine negative terminal but is wired to the same DC main negative bus.

Somewhere there will be a connection to source ground (the water surrounding the boat; earth). The ABYC defines source ground as ‘a bare conductive surface in contact with the water around the boat that is maintained at the potential of the earth’s surface’. This could be, for example, a radio or lightning ground plate. All bonding, equipotential bonding, and lightning protection circuits are connected to source ground. This may be a single point of connection inside the hull at a bolt fastening the source ground to the hull or it may be a grounding bus that in turn is connected to source ground.

In a grounded DC system, the common ground point – the negative terminal on the engine or the DC main negative bus - is intentionally connected to source ground, either directly or via the grounding bus. In fact, the same busbar can serve as the DC main negative bus and the grounding bus. In an isolated ground DC system this connection between DC negative and source ground is intentionally omitted. There must be the two busbars – the DC main negative busbar and the grounding busbar – and they must not be connected. This is the fundamental difference between the two systems.

If there is an onboard AC system, its grounding circuit must be connected to source ground. In a grounded DC system this connection can be made at the DC main negative bus (which, in turn, is connected to source ground) or directly at the grounding bus/source ground. In an isolated ground DC system, the AC grounding connection must be made at the grounding bus/source ground and not at the DC main negative bus.

Safety

An isolated DC system poses significant safety challenges. For example, let’s say we have a fault from a piece of AC equipment to DC negative. DC negative is no longer connected to source ground and as such is no longer connected to the AC grounding circuit. The AC and DC equipment on the boat may continue to work just fine with no indication of the fault until someone contacts the negative side of the DC system and becomes the path to ground, receiving a severe shock, or even being electrocuted. 

And then consider a metal boat on which the small gauge wiring run to a navigation light mounted on the pulpit has been damaged when dragging the conductors through the pulpit. The negative conductor is shorted to the pulpit and the hull. The light will work fine. But now we get another fault from a much larger positive conductor to the hull. This conductor is protected by a high-amp fuse. Because the DC system is isolated, there is not the usual low resistance connection between the hull and DC negative which would lead to a high current flow and the fuse blowing. Instead, the fault current runs through the hull to the shorted navigation light conductor and from there back to the battery. The small negative conductor is the path for the fault current back to battery negative. It melts down long before the fuse on the large conductor blows. Any conductors in a bundle with the small negative conductor also melt down. At best, a substantial rewiring job is required; at worst, the boat catches fire.

To deal with these situations, isolated DC systems require special attention to overcurrent protection and switching and are recommended (but not currently required) to have a ground fault monitoring system. This is a mechanism that warns the operator if the isolation of the system has been breached.

Double-pole circuit protection and switching

A grounded DC system typically has a battery isolation switch, overcurrent protection (fuses and circuit breakers), and equipment switches, all in the positive side of the circuits. And in fact, we do not want anything in the negative side of the circuit that might interrupt the path to battery negative. Faults that have any path to grounding circuits, and to the hull on a metal boat, have a low resistance path back to battery negative which, if the current flows are high, will blow the fuse or trip the circuit breaker on the positive side of the faulty circuit, shutting down the fault current.

An isolated ground DC system is treated differently to a grounded DC system. In this case, because there is not the low resistance path back to battery negative from grounding circuits and the hull on a metal boat, we need overcurrent protection, sized for the ampacity of the conductor being protected, on the negative side of circuits as well as on the positive side. The kind of fault we illustrated earlier from a high current circuit to a small gauge negative conductor will blow the fuse or trip the circuit breaker on the negative small gauge conductor, shutting down the fault current. The addition of a ground fault monitoring system will alert the boat operator to the ground fault.

What does this look like in practice? In general, where we would see a fuse or circuit breaker on the positive side of a circuit, we want to see a double-pole circuit breaker that simultaneously trips both the positive and negative sides of the circuit. And where we have a single pole battery or equipment switch, we want to see a double-pole switch that simultaneously breaks both the positive and negative conductors.

Exceptions

Other than requiring a double-pole battery switch in an isolated ground DC system, the ISO has no prescriptions that cover these systems although it seems fair to assume double pole circuit protection and switching is required throughout the system. The ABYC similarly has little in its existing standards that address isolated ground systems. The isolated ground sub-committee has developed a set of proposals for modification to the ABYC E-11 standard. These are currently under review and not yet incorporated in the standard. Nevertheless, it is useful to look at these proposals as they provide guidance for isolated ground DC systems.

Recognizing the difficulty of sourcing high-current double-pole switches that meet the requirements for battery isolation switches, the proposals from the sub-committee permit a single-pole battery switch (on the positive conductor) in isolated ground DC systems if both the positive and negative conductors are provided with overcurrent protection. Beyond the battery switch – i.e., on branch circuits – if the circuit has double-pole circuit breakers a single pole switch in the positive conductor to loads is permitted. However, if the branch circuit protection is provided by fuses, both the positive and negative conductors require fuses, and a double-pole equipment switch is required.

The committee considered a couple of special cases. The first one is an isolated load, such as a bow thruster or windlass, which has a dedicated battery for the single load, with the battery isolated from the rest of the boat’s DC systems, in a location where the equipment itself is electrically isolated from other electrical systems on the boat. In this case, a single overcurrent protection device and single pole battery switch, as in a grounded DC system, is permitted.

The second special case is a trolling motor connected to a dedicated trolling motor battery. This is not required to have a battery switch (either double pole or single pole) if there is overcurrent protection at the battery and a manual means of electrical disconnect (for example, a plug between the battery and trolling motor). 

If a twin-engine boat with isolated ground cranking and other engine systems has separate cranking batteries for each engine, with a paralleling circuit between the batteries, battery isolation switches need to be double pole switches, but the paralleling switch can be single pole (in the positive conductor).

AC faults

It is still possible to envisage situations in which an AC leak into isolated DC circuits creates a potentially lethal shock hazard without triggering a protective device.

The ISO 13297 standard already has general requirements that considerably mitigate this risk. The first is that all AC sources are required to have RCD protection. This applies not just to a shorepower inlet, but also to the output of a polarization or isolation transformer, an onboard AC generator, and a DC-to-AC inverter. In the event of a fault into an isolated ground DC system that fails to trip an RCD because there is no path to the source of power, the moment someone or something completes the path the RCD will trip.

The ABYC requires an ELCI (its version of an RCD – see this lesson) on a shorepower inlet but not on the output of a polarization or isolation transformer, or a generator, or an inverter. A sub-committee recommendation is to require an ELCI on these devices if an isolated DC system is installed. The language is as follows: ‘An equipment leakage circuit interrupter (ELCI) or Type A residual current device (RCD) shall be installed with or in addition to the main overcurrent protection device for each AC source if there is an isolated DC system on a boat’.

The ISO has significantly stricter separation requirements than the ABYC for AC and DC conductors. An ABYC sub-committee recommendation is to toughen the separation language in E-11 as follows: ‘AC conductors shall not be run together in the same cable, bundle, or raceway, etc. with DC conductors that are part of an isolated DC system.’

Ground fault monitoring

There are some pieces of equipment with internal bonding that may defeat the purpose of an isolated ground system, and there are other ways in which it is relatively easy to create an accidental connection from the negative side to the boat’s grounding bus or source ground. Most times this fault will not cause equipment problems and so will go undetected, perhaps for years. Given a variety of potential second fault conditions, unsafe conditions can occur, or potentially catastrophic stray current corrosion can be set in motion. A ground fault monitoring device will alarm when that first fault condition occurs.

A ground-fault monitoring device is connected to the ungrounded conductors in a circuit (the DC positive and negative in our isolated DC system) and to ground (the grounding bus or source ground). It applies a small voltage between these points, measures any current flow, and from this calculates the resistance. The ground fault monitor is measuring the resistance of the conductor insulation to ground (source ground in our case). If it detects a breakdown in this resistance, it alarms. 

In some cases, a ground fault monitor can be set to shut down the circuit if a ground fault above a certain level is detected. However, in general, we do not want the boat’s DC systems to shut down when the first fault is detected. We just want to know about it so that we can track it down and take care of it.

Ground fault monitoring is not required by either the ISO or ABYC in isolated DC systems below 50V (ISO) and 60V (ABYC). It is, however, recommended by the ABYC sub-committee. Ground fault monitoring is required for electric propulsion systems above 50V (ISO) and 60V (ABYC). 

In the absence of ground fault monitoring, it is important for boat owners and service personnel to know how to check the isolation and to periodically do this. The simplest way is with a multimeter in its ohms mode, testing from the positive conductor to the boat’s grounding bus/source ground, and from the negative conductor to the grounding bus/source ground. In both cases there should be a very high resistance. If not, there is a ground fault. If the meter blows its fuse, there is likely a double fault!

Summary

Most of us have grounded DC systems, in which case this lesson does not apply to us. However, isolated ground DC systems are becoming more commonplace. They are not difficult to install, but they do require attention to detail, including double-pole circuit protection and double-pole switching almost everywhere. 

Ideally the system will include ground fault monitoring to ensure that the isolation is maintained over time. Without ground fault monitoring, it is important for the boat owner to know how to check the isolation with a multimeter and to periodically do this.