When are principals vicariously liable for negligence committed by their independent contractors?

Generally speaking, principals are not vicariously liable for the actions of their independent contractors. For example, if you pay an independent contractor to deliver a pizza, you are not responsible for a car accident they have on their way. However, because "independent contractor" status is an effective way for principals to avoid liability, the law attempts to balance this out by limiting its effectiveness in extreme situations.

Therefore, when an independent contractor is engaged in super dangerous activities (like using explosives, or dangerous machinery), then the principal can be vicariously liable for their negligence. Likewise, where an independent contractor is engaged in duties that are vitally important to safety operations (like installing brake pads on vehicles), their negligence may also be imposed on their principal.

Put simply, imagine if employers were able to identify every risky job in their company and then, instead of hiring an employee for that position, they hired an independent contractor. Without this rule, those employers would be immune from liability and always be able to hide behind the "independent contractor" designation of those in charge of the risky part of the business. In other words, employers are not allowed to outsource their safety obligations.