Safety

• NOTE: This document was not written by an electrical engineer, its author doesn't have enough knowledge and experience to be confident enough in its content. ANYTHING DESIGNED USING THIS CONTENT SHOULD BE REVIEWED BEFORE ACTUAL USAGE. Please don't shock yourself.
• TODO: Find someone to review and validate this content.

As with all electrical systems, great care must be taken to keep the system safe and avoid injuries and equipment damage. Different systems are used for different hazards, and the characteristics of the electrical system (voltage, max current, user exposition) determines the required protections.

Isolation, earthing and residual currents

Electrical circuits must be isolated from their environment, and conductors must be isolated from other conductors. Earthing (grounding) is used to keep the exposed condutive parts close to earth potential, so that we can avoid electrical shock and electrocutions. Earthing is also useful to protect against lightning strikes, and it can improve the behaviour of residual current detectors.

Even in the case of very low-voltage circuits, there are multiple reasons why proper earthing can be required :

• Voltage doesn't kill, but current does. In some conditions, even 12V will be able to pass through a body and stop a heart or even fry everything.
• Voltage is not always guaranteed : over-voltage systems have their limits, and you may be on the wrong side of this system, in which case the over-voltage situations can get even worse as the circuit is opened.
• It can help for the operation of some over-voltage protections (surge suppressors)

There are multiple earthing schemes that can be used for DC circuits.

• TODO: Describe earthing schemes and their relation to micro-grids and their structure
• TODO: Describe RCDs, their relation to the earthing scheme and implications of their use (problems with "normal" residual current leaks, for example with cheap power supplies).

Over-current protection

An over-current condition is defined as a conductor/equipment carrying more current than it has been designed for. This situation is handled by opening the circuit, usually after a delay (to allow start-up peaks), using a thermal breaker (that opens the circuit after it has heated up to a certain level).

The thermal breakers should always be sized so that they trip before the conductors overheat.

Fault/short-circuit protection

A current fault condition is defined as a conductor/equipment carrying "abnormal" current, where the current is "abnormal" in either/both its value or its rate of change. This situation can occur in the case of a "short-circuit", but also when the circuit is accidentally connected to a different circuit (for example, accidentally connecting a DC grid to the mains). This situation is handled by opening the circuit very quickly (because the fault current can be very high), using a magnetic breaker (that opens the circuit based on the rate of change of the current).

The magnetic breakers should aways be sized so that they can properly break the maximum fault current. For this, the different fault currents must be calculated (fault current for accidental mains connection, fault current for short-circuit anywhere in the circuit, etc).

Over/under-voltage protection

An over-voltage condition is defined as two conductors having a difference in potential that is out of the specified range. This situation can occur in the case of non-managed excess supply, or rapid unloading. It must be avoided because it can damage equipments, changes the operational characteristics of other safety devices, and decreases the resistance required to avoid carrying a current (increases the risk of an electrical shock).

Under-voltage should ideally also be handled, as it can damage equipments and confuse an user/operator into thinking that a circuit is not connected to power. Under-voltage can occur in the case of lack of supply, or rapid loading.

TODO: Describe how to handle these situations, add links to the existing circuit designs