I am getting smps output +ve wrt E and -ve wrt E zero and + wrt -ve is 24 volt, how is possible?
Measuring the terminals with respect to ground is meaningless, especially for an isolated supply. If there is a transformer in your supply, and it's isolated, measuring to ground won't mean anything becasue there is a very large resistance between ground and the terminals. In this manner you can make the supply be +24V or -24V depending on how the terminals are connected. You could even take two isolated supplies and make a 48V supply by connecting them in series. 

See that pink bar dividing both sides? that isolation is the source of the problem of measuring the voltage across both sides of the SMPS.


Source: http://www.learnabout-electronics.org/PSU/psu34.php