There is no such term like employment relation in use in industrial employment. However there is an expression by name employer-employee realtionship in use in industrial employment.The expression has legal connotation in that it indicates certain features of the relationship where by a person can be treated as employee and the other person can be treated as his employer.The features of such relation ship is that one person should hire another person for some reward or remuneration9salary or wages) to do the work of the former under his supervision and control.You can also call it a master-servant relationship. From such relationship will flow the terms of employment and conditions of servce of the employees.Thus this relationship is the foundation to ditermine the rights and obligation between the employer and the employee. It has no other purpose beyond this.
Industrial relation is a much wider term and iis used to refer to the relations between the employer and the employees collectively with reference not only to the terms of employment and conditions of sevice of employees but also any issue that affects their terms of employment and conditions of servcie. thses relations are governedand guided by negotiations or collective bargaining, culminating in settlements on such matters. For example, negotiations on such terms of employment like working hours or leave and such conditions of servcie like bonus or evn issues like closure etc. can fall under the ambit of industrial rellations. Thus employer- employee relationship is more an individual relationship between an empoyer and the employee and Industrial relations is mor a collective relationship between employer on one side and the employees collectively on the otherside.Thus industrial relatioship indicates working climate in an industry.
Human relations is infinitely wider term as to cover the whole gamut of the conduct of a human being in his relation with others and is characterised by such qualities like empathy, sympathy,kindness, patience, hearing others and rational thinking etc. and can influence all other relations like employer-empoyee, industrial relations,social relations or even relations betwenn one nation and the other.
Hope this helps to clarify the difference in scope and meaning of these expressions.
B.Saikumar