A gross attack is one heavy impact and alarms immediately; pulse counting accumulates smaller impacts and alarms when enough occur within a set time.

The two exist because attacks come in both forms. A vehicle through a shutter is one event; drilling a safe is hundreds of small ones.

The gross threshold has to sit above the heaviest legitimate impact the structure receives, such as a door being slammed.

The pulse count and its time window have to sit above normal background activity while still catching sustained work.

Both are adjustable, and both should be set from observation of the actual site rather than from a default.

Getting one right and leaving the other at factory settings is the usual reason a shock detector is either useless or intolerable.

On solid structure - a frame, a wall, the body of a safe - within the manufacturer's stated protected radius, and never on a loose panel.

The sensor responds to vibration in whatever it is attached to, so a flexible or rattling substrate produces constant activity.

Manufacturers state a protected radius, typically a couple of metres on suitable structure, and it reduces on materials that damp vibration.

Fixing method matters: a detector screwed firmly to the substrate transmits the vibration, one on an adhesive pad may not.

On a door, the frame is usually better than the leaf because the leaf moves in normal use.

Different materials transmit differently - masonry, steel and timber all behave unlike one another, so a position that works on one wall may not on the next.

Ordinary building vibration: slamming doors, shutters in wind, traffic, plant, deliveries and weather.

The detector cannot distinguish an impact from an attack by anything other than magnitude and pattern.

Roller shutters are the most common problem, because wind moves them constantly and they are exactly what needs protecting.

Nearby heavy traffic, rail lines and industrial plant produce continuous background vibration that consumes the pulse count.

Hail and heavy rain on metal cladding and rooflights are a seasonal cause.

The fix is threshold setting and mounting position. A site with genuinely constant vibration may not suit shock detection at all.

Safes, cash machines, strongroom walls, roller shutters, external doors and glazing where acoustic detection is unsuitable.

Safes and cash machines are the classic application, because the attack is prolonged and physical.

Roller shutters and external doors benefit from detection before entry rather than after.

Strongroom and vault walls are protected against cutting and drilling through the fabric rather than through an opening.

Laminated glazing suits a shock sensor better than an acoustic detector, because laminated glass may crack without producing a clear fracture sound.

Insurers frequently specify shock protection on high-value stock rooms, and their requirement may dictate the type and the placement.

With a controlled tap at the furthest protected point, using the manufacturer's test method - and re-tested whenever the structure changes.

Most detectors have a test mode with a local indicator showing when a pulse is registered, which allows the sensitivity to be set without triggering the system.

Test at the edge of the protected radius rather than beside the sensor, because that is where coverage fails.

Record the settings and the test result so a later change can be compared against a baseline.

Re-test after any structural work, a new door, a replaced shutter or a change to the surrounding plant.

Seasonal re-checking is worth doing on external applications, since wind and temperature change the background.