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Apart from relay systems, interface units not forming part of more complex items of apparatus were very rare in intrinsic safety, and for analogue circuits non-existent, until the development of the shunt diode safety barrier which first received recognition in 1964. The barrier device is now 21 years old and its limitations well known to us. In the last two or three years alternative interface units called generally ‘isolated interfaces’ have been developed and much is claimed for these — indeed there are those who claim that isolated interfaces are as big a technological breakthrough as was the barrier device before it. The objective of this article is to compare the isolated interface with the barrier and attempt to demonstrate just what advantages it possesses and how much of a breakthrough it really is.