Air filters: What is the purpose of a safety filter?

The Filter Guys

What is the purpose of a safety filter? Safety filter, secondary filter, inner filter; there are many names, we prefer to call it a safety filter.

A safety filter backs up the primary filter, and protects the clean air ducting while the primary is out of the housing during service. The engine should never be run with only a safety filter in place.

The safety filter is NOT a spare filter! Its purpose is to protect the engine if something goes wrong with the primary (main) filter. Until then, all it does is take up space and add a little bit of restriction.

What can go wrong with the primary filter?

It may have hidden damage from shipping that you missed when you inspected it before installation; damage from cleaning (if you carry out such an unrecommended procedure); maybe even a manufacturing glitch; mis-installation by the mechanic (but this never happens, right?); a ‘will-fit’ that doesn’t quite fit; or it’s the wrong part altogether and doesn’t fit at all into the housing (but mechanics would never try this, would they?)

All of these things happen more often than any of us like to think. We determined many years ago that a really good way to provide additional protection for large engines operating in heavy dust conditions was to always offer and for you to always use a safety filter.

There are some characteristics about the safety filter which you should be aware of, so you understand what it does and doesn’t do for your engine:

  • is smaller than the main filter, with less media.
  • is more open and less efficient for lower restriction.
  • has less contaminant-holding capacity so that it will plug rapidly if any engine-damaging dust particles get past a damaged primary filter.

A safety filter offers protection against mistakes when fitting and reduces the need to open the clean air side of the system by 66% (they usually only need changing once every three main filter changes) compared to systems without a safety filter. They are an excellent insurance against contaminants entering your engine and upon a critical failure of the main element can save your engine from serious damage.