Nippon Hakuyo Electronics NHE-Oki FD-6557 Fixed Temperature Heat Detector 70degC Addressable The Nippon Hakuyo Electronics NHE-Oki FD-6557 is a low profile addressable fixed temperature heat detector, 70degC. Oki FD-6557 marine fixed temperature heat detector detects abnormally high (static) temperature and provides a fast response that depends on the absolute temperature reaching 70°C. FD-6557 rated voltage/current DC28V / 50mA. Warning current 2mA and operation voltage range: 20V-30V.