Loop Info-Q-2-16
8
SECTION 2 - LOOP INSTALLATION GUIDELINES
In order to have a loop detection system operate as a reliable, high performance system, it is necessary to pay careful attention
to the loop installation. The use of proper installation techniques and the proper type of wire can reduce frustration, aggravation,
and unnecessary service calls.
Depth of Saw Cut - The depth of the saw cut will vary depending on the number of wire turns, typically 1 1/2” for 1-3 turns and 2” for 4-6
turns. The backer rod should be at least 1 inch below the surface of the pavement. The saw cut must be free from loose debris which can
damage the wire insulation. If the saw cut is too deep, the roadway under the saw cut may crack over time, creating a possible path for water
into the road bed material. One guideline to consider would be to try not to exceed 50% of the overall pavement depth when cutting a slot for
the loop wire. Greater depth should be used in softer pavement materials to protect the loop wire against damage from surface erosion and
wear.
Shape of Loop - The loop is typically cut in a rectangle with 45° angled corners that eliminates the sharp edges that can eventually cut through
the wire insulation. DO NOT allow the rectangular saw cuts to connect in the corners or the triangular pieces that are created will eventually
break free from the roadway. Cut an exit slot with NO sharp angles for the wires where they leave the loop saw cut.
Types of Loop Wire - Moisture in the loop’s wire insulation can cause significant changes in the dielectric constant resulting in excessive loop
(frequency) drift. Choose a wire insulation that is impervious to moisture. Typically 18 AWG Cross Linked Polyethylene (XLPE) insulated
wire is used with COLD sealant because of the ease of installation. It is rated for direct underground burial which makes it very resistant
to moisture absorption and provides good abrasion resistance. 18 AWG Polyvinyl Chloride (PVC) insulation (TFFN, THHN, THWN) should
ONLY be used with HOT sealant that will encapsulate the wires and protect them from all moisture. This wire is not rated for direct under-
ground burial and will absorb moisture if not completely protected by the hot sealant. DO not use cold sealant with this type of wire because it
will not completely encapsulate the wires. Any voids in the sealant will allow moisture next to the wire insulation that can cause a loop failure
in time. Either type wire must be wound with one continuous length, No splices are allowed.
Backer Rod - The loop wires must be held securely in the bottom of the slot by means of a plastic foam type material called
“Backer Rod”. The recommended method of holding the loop wire in the slot is to use a series of one (1) inch long pieces of backer rod
spaced approximately one (1) foot apart along the entire length of the saw cut and exit slot. Make certain to press the backer rod tightly into
the slot. Use a wooden stick or other blunt instrument to avoid potential damage to the loop wire. If the backer rod is not pressed tightly into
the slot, the loop wires will be loose in the slot and can cause false calls due to vibration or sudden movement.
Cold or Hot Sealants - Cold sealant is typically preferred because it is easier to use than hot sealant. It is pliable and will “Give” with tempera-
ture changes in the type of roadway used (Typically asphalt or concrete). When properly applied, the cold sealant should completely cover the
loop wires and backer rod and level with the top of the roadway. Since the sealant forms a barrier between the wire and the environment, it is
essential that the wires are completely covered. There must be NO voids where water can collect and freeze within the slot. The freeze/thaw
cycling will eventually push the loop wires up and out of the slot where the voids exist resulting in a loop failure. Hot sealant must completely
surround the loop wires and backer rod to protect them from moisture. Very close attention must be paid with this type of installation because
any voids in the sealant can allow moisture next to the wire insulation causing a loop failure in time. Hard setting epoxies should not be used
in softer material like asphalt.
Lead-In Wire - The lead-in wire is a continuation of the loop wire (No splicing) and is typically used when the distance between the loop and
the detector is 100 ft or less. It must be tightly twisted together with a minimum of six turns per foot.
Lead-In Cable - If a lead-in cable is used (Typically for distances greater than 10 ft between the loop and the detector), it should be a shielded,
twisted pair with a high-density polyethylene insulation. The shield should be floated, left unconnected and insulated at the spliced end and
grounded to an earth ground (Operator’s ground) at the detector end. Any other grounding arrangements can cause erratic system operation.
(See page 6)
Splice Connections - 1. All splices must be soldered - do not use wire nuts or crimp connectors because the output from the loop detector to
the loop is at very low voltage. This means that there is not enough power to go through a weak connection. 2. Each splice point must be
protected with a moisture proof seal. No splices are allowed in the loop itself. Failure to observe these precautions are the two most
common causes of loop related problems.
Exit Slot - Where the loop wires leave the loop saw cut (NO sharp angles), they must be tightly twisted together with a minimum of six turns
per foot. Use tape on the twisted end to hold the wires together. This prevents false calls from movement between the wires.
After determining the size, placement and number of turns of the loop:
(See below text and illustrations on the next page)
Saw Cut Loop Guidelines
