What does N+1 mean? 

As a professional electrical engineer, people sometimes remind me that what I take for granted is a new concept for other people.  Following a recent admonishment, I should make more effort at covering introductory level topics.

The term 'N+1' relates to redundancy and simply means that if you required 'N' items of equipment for something to work, you would have one additional spare item.  If any one item of equipment breaks down, every thing can still work as intended.  For example if you need two UPS for a small data centre, you would install three to have N+1; if you have a water pump in your building your would install a second one for N+1; if you require three transformers to power your facility you would install four for N+1.

People sometimes use the term 'N-1' to mean the same thing (i.e. if you lose one item you can still work).  This is a less common.

This type of terminology is often expanded.  '2N'  (sometimes referred to as 1+1) means you have twice as many items as you need - in the data centre example you would install four UPS.  The pumping example is already 2N (in addition to being N+1).  Less common, but possible are things like 'N+2", '2N+1", etc.

A final word on topic levels.  I do try to mix my posts at varying levels so that anyone visiting the site will find something of value.  If while browsing the site you have an idea for a short post on a particular topic or  want some thing explained in simpler terms or in greater detail;  just let me know.  You can leave comments on related posts or use the Contact Us page to send in a request.  While there is no guarantees, I will try to address as many requests as I can.