Installing a reliable ground loop can be challenging at the best of times.
There are so many critical points to consider such as the size and depth of the loop, the number of turns per side and the best material to construct it from. The good news is that, by sticking to the eight guidelines below as closely as possible, you too can install a ground loop that is reliable and efficient and can be used for any number of vehicle access applications, from free-exit and closing loops, to safety loops and arming loops for access control devices.
- Construct the loop and feeder wire from cross-linked polyethylene insulated multi-stranded copper wire with a minimum cross-sectional area of 1.5mm². This type of cable has proven to generate excellent inductance and also provide good immunity to outside noise. If, however, the loop feeder is installed in a location where it may pick up electrical noise, it is best to use screened cable and earth the screen at the loop detector. The feeder should be twisted at a rate of at least 20 turns per metre to improve reliability.
- Joints in the wire are not recommended, but where required must be soldered and made waterproof. The reason for this is that joints may cause larger impedances (electrical resistance) which will result in erratic and unreliable operation of the ground loop.
- The loop should be either square or rectangular in shape with a minimum distance of 1m between opposite sides.Two to six turns of wire are typically used in the loop, depending on the size of the loop. The schedule below should give you a rough idea of how many turns are needed for each individual loop size.
Number of turns needed for loop size
Loop perimeter (metres) Number of turns 3-4 6 4-6 5 6-10 4 10-20 3 >20 2
- When two loops are laid in close proximity to each other, it is recommended that different numbers of turns are used in each loop to prevent cross-talk. Cross talk describes the interference between two adjacent loops, and can cause reliability issues. To minimise cross-talk, adjacent loops should be at least two metres apart, and on different frequency settings.
- As a rule of thumb, and for improved reliability of your inductive loop, it is always recommended that the loop is preformed and enclosed in conduit. Remember, a loop is still an electrical system and the presence of water will make the system completely unreliable. In addition, cars travelling over the loop all day long will cause significant vibration and possibly cause the loop to move if it is not shielded with conduit.
- Where a pre-formed loop is not practical, slots should be cut into the road using a masonry cutting tool. A 45° cut should be made across the corners to prevent damage to the wire on the corners. The slot should be about 4mm wide and 30mm to 50mm deep. Remember to extend the slot from one of the corners to the roadside to accommodate the feeder. After the loop and feeder wires have been placed in the slot, the slot must be filled with an epoxy compound or bitumen filler.
- Install the accompanying loop detector in a waterproof housing and as close the ground loop as possible
It is also recommended that you make use of a reliable and sensitive loop detector. Decide on how many channels will be needed, whether a 12V or 220V detector is better suited for the application, then contact your nearest access control specialist and ask them what detector they recommend.