A furnace comprising: - a furnace shell defining a chamber on the inside of the shell; - a roof; and - at least a first elongate electrode and a second elongate electrode extending through openings (30, 32) in the roof into the chamber, the electrodes (24, 26) being connected to a furnace power supply via an electric circuit comprising a circuit breaker; and - a stray arc protection system comprising: ∘ an electromagnetic signal imaging device mounted externally of the shell, the imaging device being mounted and configured to generate data relating to electromagnetic emissions in a predetermined zone (44, 46) of interest externally of the shell and where a stray-arc may develop; ∘ a controller comprising a processor executing an application specific program and which controller is connected to the imaging device to receive the generated data; ∘ at least one of the imaging device and the software being configured to mask in data relating to the zone (44, 46) of interest and to mask out data relating to adjacent regions which are not of interest; o the processor being configured to: process the generated data into intensity data; compare the intensity data to predetermined threshold intensity data which is indicative of a stray-arc forming; and to generate at an output an output signal when the intensity data exceeds the threshold intensity data. - a furnace shell defining a chamber on the inside of the shell; - a roof; and - at least a first elongate electrode and a second elongate electrode extending through openings (30, 32) in the roof into the chamber, the electrodes (24, 26) being connected to a furnace power supply via an electric circuit comprising a circuit breaker; and - a stray arc protection system comprising: ∘ an electromagnetic signal imaging device mounted externally of the shell, the imaging device being mounted and configured to generate data relating to electromagnetic emissions in a predetermined zone (44, 46) of interest externally of the shell and where a stray-arc may develop; ∘ a controller comprising a processor executing an application specific program and which controller is connected to the imaging device to receive the generated data; ∘ at least one of the imaging device and the software being configured to mask in data relating to the zone (44, 46) of interest and to mask out data relating to adjacent regions which are not of interest; o the processor being configured to: process the generated data into intensity data; compare the intensity data to predetermined threshold intensity data which is indicative of a stray-arc forming; and to generate at an output an output signal when the intensity data exceeds the threshold intensity data. ∘ an electromagnetic signal imaging device mounted externally of the shell, the imaging device being mounted and configured to generate data relating to electromagnetic emissions in a predetermined zone (44, 46) of interest externally of the shell and where a stray-arc may develop; ∘ a controller comprising a processor executing an application specific program and which controller is connected to the imaging device to receive the generated data; ∘ at least one of the imaging device and the software being configured to mask in data relating to the zone (44, 46) of interest and to mask out data relating to adjacent regions which are not of interest; o the processor being configured to: process the generated data into intensity data; compare the intensity data to predetermined threshold intensity data which is indicative of a stray-arc forming; and to generate at an output an output signal when the intensity data exceeds the threshold intensity data.