TT system - maximum earth fault loop impedence?

[ainsdale_gas]

I've recently completed a part p defined scope course but all of our testing was on TNS or TNCS systems, in our house we have a TT system and in the OSG I can't find much information on the maximum earth fault loop impedence, just says that it is likely to be higher than their TN tables? I might ring the IEE helpline tomorrow.
What do you use for your maximum figures on TT systems?

 

[muzza1]

Great lads, very informative. Thanks.