To monitor two pieces wrist strap at the same time. * Earth system of body wrist strap in normal operation;no noise,green light on.Earth system of body wrist strap in circuit-broken status:buzz noise,red light glittering Features: Designed to monitor resistance of the operator's path to ground. It comes with a built in audible alarm and LED, triggering either when the operator falls out of spec, becomes disconnected, or ground is lost. Monitor 2 wrist straps at the same time.