Designing Electrical Safety Systems For Off-Grid Renewables

In a previous article, we explored the practicality of wind power in the interest of leveraging a variety of renewables as part of an off-grid campervan conversion project. Given the rapidly growing scope of the project so far, the next logical step should be to emphasise the safety considerations needed for renewables power systems past a certain size and what safeguards we can put in place to protect ourselves when living off-grid. This article, therefore, will focus on the components and methods we can employ to guard against shorts, shocks, surges and ground leaks while exploring the characteristics of each safety component in the context of renewables by applying them to our project.

Components and Parts

• RCD
• MCB
• Fuse Holders
• Cartridge Fuses
• Isolator Switch
• Fuse Board Enclosure
• Consumer Unit Enclosure

Fuses VS Breakers

There are many different types and variations of electrical safety components currently in use for domestic, commercial and industrial environments. Understanding the requirements of our particular system is therefore critical in assigning the appropriate safety devices. A typical renewable energy system combines both low-voltage DC and domestic mains AC power profiles, which presents an excellent opportunity to explore the difference between the two main categories of electric safety components and why they are conducive to each application.

Fuses

Fuses are the simplest form of electrical safety device available and are easy to assign based on a known maximum current rating, whether this is AC or DC. Fuses are single-use devices that are constructed using a thin conductive core that acts as the weak point in any system and breaks when exposed to excessive current. This simple design makes them cheap and easy to install, especially to protect DC equipment like our renewable energy sources.

10 x 38mm cartridge fuses

The electrical configuration of the renewable system will dictate what fuse size will be appropriate in each case, for example, the nominal current for a 100W solar panel is 5.7 amps so a 6A fuse would protect against any surges or shorts. Connecting two or more panels in series as part of an array would not affect the current rating requirements, but a larger fuse would be required for panels connected in parallel. Alternatively, you could connect a fuse to each panel in a parallel array so any single-point failure mode would not inadvertently bring down the whole power system.

Breakers

Breakers are primarily designed to protect AC mains circuitry in the event of shorts, surges and ground-leakage when connected to the national grid. As we are working off-grid however, these devices can be used to protect the output taken from the inverter stage of our renewable setup, which converts stored DC power into the mains AC that is found in every household wall socket.

30mA RCD

The standard UK power socket is one of the safest in the world due to its design that features an extra pin that is connected directly to earth. This addition ensures a safe failure-mode in any appliance or household object that may otherwise have exposed metal components sitting at live voltages. An RCD or Residual Current Device is a component that can be used to monitor any discrepancies caused by currents leaking unimpeded to earth. Applying residual current protection to an off-grid system is simply a matter of finding a common earthing point. Fortunately, we can use our campervan chassis as an earth for all our equipment which gives electrical current an easy route back to the battery.

2A B-type curve MCB

The other main type of circuit protection device is the MCB or Mini Circuit Breaker, which protects against shorts and surges by disconnecting the circuit above a fixed current threshold in much the same way as a fuse. MCBs are also rated by their responsiveness to surges, where a Type B domestic curve will trip faster than a Type C commercial or Type D industrial curve. These characteristics make MCBs a lot more flexible than fuses and can be reset once any issues have been rectified.

The combination of both RCD and MCB will give excellent protection against any mains power failure mode and ensure the user and the system are protected at all times. The next step, therefore, is to apply this new knowledge in the interest of keeping our renewable energy system safe once complete.

Building a Fuse Box and Consumer Unit

To protect the respective DC and AC components of our renewable system, we will need to construct the relevant protection circuits. To do this, we will be assembling a simple fuse box and consumer unit that can both be accessed safely and serviced easily by the end-user.

Fuse box

The fuse box will be designed to protect our incoming solar and wind power from excessive currents caused by potential short circuits or surges. We will be using two cartridge fuse holders, one for each renewable source, mounted in a plastic DIN rail enclosure with two knockouts for entry and exit of the relevant cables.

Wiring the fuse box to incoming and outgoing DC

We can connect one of the supply cables from each source to their respective fuse holder by threading them through the two knock out holes on the DIN rail enclosure and protect them from damage with two rubber grommets. The flexibility of these enclosures means we also have the option of using a round conduit to tidy up any loose cables while protecting them from damage.

Fuse box assembly showing cartridge fuse

Once we have assembled the enclosure, we can insert the fuses that correspond to the maximum rating on our renewable’s sources. The current solar array needs protection from currents above 6A while our wind turbine should automatically furl itself out of the wind above 12A. It’s then a simple matter of inserting the fuse in the corresponding fuse holder, making sure to isolate the power beforehand to negate the risk of heat burns and electrical arcing.

Consumer Unit

In contrast with the fuse box, the consumer unit will be designed to protect our outgoing AC mains voltages that will supply a range of domestic appliances. Installing a consumer unit not only protects the renewable system but also stops shorts and surges damaging any potentially expensive equipment like phones and laptops while also mitigating the risk of electrical fires caused by hot wires.

This is especially important in an enclosed space like a campervan where any excess cable currents or floating chassis voltages are more likely to cause combustion in the vicinity of wood, gas and even diesel fuel. To protect against these failure modes, we will be building a simple consumer unit with an isolator switch, RCD and single MCB that are sensitive enough for a motorhome environment.

Consumer unit with isolator, RCD and MCB

The renewable system uses an inverter to derive the mains supply from the leisure battery, which is rated at 1000W. As this power rating is relatively low, we will be using a standard 30mA RCD to detect any potential leaks to the common earth, while using a high sensitivity 4A B-type MCB that will trip at over 920W and leave a margin of safety in our mains circuit. Alternatively, we could use a 2A B-type MCB for 500W rated inverters.

Wiring the consumer unit with incoming and outgoing mains AC

Assembling the consumer unit is a very simple process. The isolator switch and RCD are connected to the incoming mains in series, using both live and neutral wires to detect current leaks. The MCB is then connected last using a single live wire so as to detect current faults in-line with the outgoing mains circuit. The incoming and outgoing earth wires can either be connected to the chassis or bypassed through the consumer unit enclosure using a terminal block or crimp splice. Once this is complete, we can connect our outgoing circuit to the mains sockets around the van and plug in our appliances knowing that they are safe from electrical faults and our van is safe from fire.

Protection circuits ready for installation

In a later article, we will focus on the considerations needed to install our new safety devices into the campervan and how to reverse engineer our electrical system to match the rating of our renewables.

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