I have a robin kts 1620 multifunction meter.The unit is not fitted with a d,lock when testing zs it will of cause trip the rcd protecting that circuit but not before i get a zs reading on the meter i have tried twice on both cycles 0 degrees and 180 degrees and they both give the same readings on there cycles can i trust this reading or should i do the onms law as well to confirm has anyone else got a robin meter and has any ideas can i have a d,lock put in or can you buy any accessorie to help??