ESR is not relevant for calculating superconductor inrush currents or other peak currents. Negative temperature coefficient (NTC) thermistors and fixed resistors are often used to limit inrush current. Another way to avoid the transformer inrush current is a "transformer switching relay". Capacitive loads will then be subject to high inrush currents upon power-up. Fixed resistors are also widely used to limit inrush current. A switch designed for an incandescent lamp load can withstand this inrush current. The first active PFC circuits just delayed the inrush. As a result the inrush current is reduced. The cause was a sudden inrush of water at a depth of . The miners who escaped the inrush of water made similarly good decisions. Inrush current can also be reduced by inrush current limiters. Inrush limiting resistors are much cheaper than thermistors.