A Press Release issued by the American Academy of Pediatrics, concluded that although side of step entrapments were common, combplate entrapments were the most prevalent. (Click here to go to the AAP and review the press release) 

Often people think that if something gets caught, the unit will stall. These machines are designed to carry extremely large loads with minimal fluctuation in speed, consequently they are unlikely to stop unless a safety device has been activated.

The potential for the accidents can be greatly reduced by lubricating the skirt panel and maintaining a minimal gap between the step and the skirt. The A17.1 code requires that the gap is less than 3/16" on either side (a sum of 3/8" for both sides). In comparison the Canadian code requires that the gap should not exceed 3/16" on both sides with a sum not exceeding 1/4". Some newer escalators specify as small as 1/16" on each side. 

A17.1a 1997, rule 802.3e - Clearance between skirt and step

The clearance on each side of the steps between the step tread and the adjacent skirts panel shall not be more than 3/16" (4.8mm).
 

Montgomery-KONE E5000 series, sales literature

New patented step guidance system reduces the clearance between the steps and the skirt panels to a nominal 1/16".
 

A17.2.3b 1997, item 1.17 - Skirt panels

Check the Clearance between the skirt panels and the step using a thickness gauge or by laying a small rule on the edge of the step to read the distance. Several steps should be checked through their entire travel. The allowable clearances are as follows:

(a) A17.1-1955 through A17.1d-1970: not more than 3/16" (4.8mm) with a total of both sides not more than 1/4" (6.4mm), except where skirt obstruction devices are installed at the lower entrance for escalators installed under the A17.1-1965 through A17.1d- 1970.

(b) A17.1-1971 through A17.1 1979: not more than 3/8" (9.5mm) on each side.

c) A17.1-1980 and later editions and A17.3: not more than 3/16" (4.8mm) on both sides.
 

Other devices such as the Safety Side Plate are designed to virtually eliminate the gap, and fill the void on the sides of the step. Brush guards, which have been used extensively in Europe, have also been quite successful.

Maintenance issues such as misaligned / stretched step chains, defective step rollers, and poorly maintained skirt panels often cause enlarged gaps, increasing the potential for accidents.

In addition to maintaining a minimal clearance the code also calls for skirt switches. These are located near the comb section and are designed to prevent a side of step entrapment from becoming a combplate amputation.

The potential for accidents can be almost eliminated by ensuring adequate mesh at all times. The A17.1 code requires that the tips of the combplate teeth are below the surface of the step tread. This provides a minimal requirement. Most manufacturers specify better than 3/16" mesh which is similar to most international codes.

A17.1a 1997, rule 802.6a - Combplates

...(2) The comb teeth shall be meshed with and set into the slots in the tread surfaces so that the points of the teeth are always below the upper surface of the step treads.
 

Otis J Escalator Installation Manual (10-1-77)

Subject: Lower landing floor plates

... A distance of 4mm (approximately) should be maintained between the bottom of the combplate and the root of the step tread. ...

The standard test for mesh is to hold a business card at the interface between the step and comb to see if it is drawn into the machine. This method applies no load to the step and therefore will not identify things like cracked steps and worn or soft rollers and bushings. 

A17.2.3b 1997, item 1.7 - Combplates

Inspect the comb for missing teeth and proper mesh with each step tread. Verify proper mesh with each step tread by holding a sheet of stiff paper or a business card where the teeth mesh with the step tread, while standing well back from the plate to prevent forcing the plate down. Use of an extension with an alligator clip to hold the card is recommended.

In addition to maintaining a minimum mesh the code also calls for comb-step impact devices which are designed to react to a horizontal impact or vertical upthrust of the comb section; and step level devices which are designed to detect a step drop of 1/8". 

Comment: If the comb-step mesh is set to the minimum allowable by code (marginally better than 0"), then a gap could exist, between the bottom of the comb teeth and the step surface, of up to 1/8" before activation of the step level device would occur.

It should also be noted that the step level device is only required to detect drops on the riser end of the step.